diff --git a/collects/mzlib/a-signature.rkt b/collects/mzlib/a-signature.rkt
index 423abb7..28026a6 100644
--- a/collects/mzlib/a-signature.rkt
+++ b/collects/mzlib/a-signature.rkt
@@ -1,11 +1,11 @@
 (module a-signature mzscheme
-  (require-for-syntax "private/unit-compiletime.ss"
-                      "private/unit-syntax.ss")
-  (require "unit.ss")
+  (require-for-syntax "private/unit-compiletime.rkt"
+                      "private/unit-syntax.rkt")
+  (require "unit.rkt")
   
   (provide (rename module-begin #%module-begin)
            (all-from-except mzscheme #%module-begin)
-           (all-from "unit.ss"))
+           (all-from "unit.rkt"))
 
   (define-for-syntax (make-name s)
     (string->symbol
diff --git a/collects/mzlib/a-unit.rkt b/collects/mzlib/a-unit.rkt
index 3e00402..e0e9446 100644
--- a/collects/mzlib/a-unit.rkt
+++ b/collects/mzlib/a-unit.rkt
@@ -1,10 +1,10 @@
 (module a-unit mzscheme
-  (require "unit.ss")
+  (require "unit.rkt")
   (require-for-syntax syntax/kerncase)
   
   (provide (rename module-begin #%module-begin)
            (all-from-except mzscheme #%module-begin)
-           (all-from "unit.ss"))
+           (all-from "unit.rkt"))
   
   (define-for-syntax (make-name s)
     (string->symbol
diff --git a/collects/mzlib/contract.rkt b/collects/mzlib/contract.rkt
index 8516d28..e9a8146 100644
--- a/collects/mzlib/contract.rkt
+++ b/collects/mzlib/contract.rkt
@@ -5,23 +5,23 @@
 ;;  provide arrow contracts from our local copy
 ;;
 
-(require "private/contract-arrow.ss")
-(provide (all-from-out "private/contract-arrow.ss"))
+(require "private/contract-arrow.rkt")
+(provide (all-from-out "private/contract-arrow.rkt"))
 
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;;
 ;;  provide contracts for objects
 ;;
-(require "private/contract-object.ss")
-(provide (all-from-out "private/contract-object.ss"))
+(require "private/contract-object.rkt")
+(provide (all-from-out "private/contract-object.rkt"))
 
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;;
 ;; old-style define/contract
 ;;
 
-(require "private/contract-define.ss")
-(provide (all-from-out "private/contract-define.ss"))
+(require "private/contract-define.rkt")
+(provide (all-from-out "private/contract-define.rkt"))
 
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;;
@@ -38,7 +38,7 @@
          racket/contract/private/basic-opters)
 
 (provide 
- opt/c define-opt/c ;(all-from "private/contract-opt.ss")
+ opt/c define-opt/c ;(all-from "private/contract-opt.rkt")
  (except-out (all-from-out racket/contract/private/ds)
              lazy-depth-to-look)
  
diff --git a/collects/mzlib/date.rkt b/collects/mzlib/date.rkt
index 88db41a..7274248 100644
--- a/collects/mzlib/date.rkt
+++ b/collects/mzlib/date.rkt
@@ -1,7 +1,7 @@
 
 (module date mzscheme
 
-  (require "list.ss")
+  (require "list.rkt")
 
   (provide date->string
 	  date-display-format
diff --git a/collects/mzlib/deflate.rkt b/collects/mzlib/deflate.rkt
index c0f89d1..5b237d3 100644
--- a/collects/mzlib/deflate.rkt
+++ b/collects/mzlib/deflate.rkt
@@ -12,7 +12,7 @@
 
   (provide deflate gzip-through-ports gzip)
 
-  (require "unit200.ss")
+  (require "unit200.rkt")
 
   (define (vector-ref* v i)
     (let ([r (vector-ref v i)])
diff --git a/collects/mzlib/defmacro.rkt b/collects/mzlib/defmacro.rkt
index f0e7509..8f0e8d4 100644
--- a/collects/mzlib/defmacro.rkt
+++ b/collects/mzlib/defmacro.rkt
@@ -1,7 +1,7 @@
 
 (module defmacro mzscheme
   (require-for-syntax syntax/stx
-		      "private/dmhelp.ss")
+		      "private/dmhelp.rkt")
 
   (provide define-macro
 	   defmacro)
diff --git a/collects/mzlib/etc.rkt b/collects/mzlib/etc.rkt
index 3694123..bb72508 100644
--- a/collects/mzlib/etc.rkt
+++ b/collects/mzlib/etc.rkt
@@ -10,13 +10,13 @@
                build-vector
                compose)
          (rename scheme/base base-else else)
-         "kw.ss")
+         "kw.rkt")
 
 (require-for-syntax syntax/kerncase
                     syntax/stx
                     syntax/name
                     setup/main-collects
-                    "private/stxset.ss")
+                    "private/stxset.rkt")
 
 (provide boolean=? symbol=?
          identity
diff --git a/collects/mzlib/include.rkt b/collects/mzlib/include.rkt
index b8f4606..8050a00 100644
--- a/collects/mzlib/include.rkt
+++ b/collects/mzlib/include.rkt
@@ -1,8 +1,8 @@
 
 (module include mzscheme
   (require-for-syntax syntax/stx
-		      "private/increader.ss"
-		      "cm-accomplice.ss")
+		      "private/increader.rkt"
+		      "cm-accomplice.rkt")
   (require mzlib/etc)
 
   (define-for-syntax (resolve-path-spec fn loc stx build-path-stx)
diff --git a/collects/mzlib/port.rkt b/collects/mzlib/port.rkt
index c588143..a7a0337 100644
--- a/collects/mzlib/port.rkt
+++ b/collects/mzlib/port.rkt
@@ -3,7 +3,7 @@
 (require (for-syntax racket/base)
          racket/contract/base
          mzlib/list
-         "private/port.ss")
+         "private/port.rkt")
 
 (define (input-port-with-progress-evts? ip)
   (and (input-port? ip)
diff --git a/collects/mzlib/private/contract-arr-obj-helpers.rkt b/collects/mzlib/private/contract-arr-obj-helpers.rkt
index 5123ffd..e42183f 100644
--- a/collects/mzlib/private/contract-arr-obj-helpers.rkt
+++ b/collects/mzlib/private/contract-arr-obj-helpers.rkt
@@ -5,7 +5,7 @@
 (require (for-syntax scheme/base))
 (require (for-template scheme/base)
          (for-template racket/contract/private/guts)
-         (for-template "contract-arr-checks.ss"))
+         (for-template "contract-arr-checks.rkt"))
 
 (provide make-/proc ->/h ->*/h ->d/h ->d*/h ->r/h 
          ->pp/h ->pp-rest/h 
diff --git a/collects/mzlib/private/contract-arrow.rkt b/collects/mzlib/private/contract-arrow.rkt
index 038951a..74af99a 100644
--- a/collects/mzlib/private/contract-arrow.rkt
+++ b/collects/mzlib/private/contract-arrow.rkt
@@ -2,11 +2,11 @@
 
 (require racket/contract/private/guts
          racket/contract/private/opt
-         "contract-arr-checks.ss")
+         "contract-arr-checks.rkt")
 (require (for-syntax racket/base)
          (for-syntax racket/contract/private/opt-guts)
          (for-syntax racket/contract/private/helpers)
-         (for-syntax "contract-arr-obj-helpers.ss")
+         (for-syntax "contract-arr-obj-helpers.rkt")
          (for-syntax syntax/stx)
          (for-syntax syntax/name))
 
diff --git a/collects/mzlib/private/contract-object.rkt b/collects/mzlib/private/contract-object.rkt
index 66cc2c5..d877bac 100644
--- a/collects/mzlib/private/contract-object.rkt
+++ b/collects/mzlib/private/contract-object.rkt
@@ -1,12 +1,12 @@
 #lang racket/base
-(require "contract-arrow.ss"
+(require "contract-arrow.rkt"
          racket/contract/private/guts
          racket/private/class-internal
-         "contract-arr-checks.ss")
+         "contract-arr-checks.rkt")
 
 (require (for-syntax racket/base
                      racket/contract/private/helpers
-                     "contract-arr-obj-helpers.ss"))
+                     "contract-arr-obj-helpers.rkt"))
 
 (provide mixin-contract
          make-mixin-contract
@@ -26,7 +26,7 @@
     (define (obj->pp-rest/proc stx) (make-/proc #t ->pp-rest/h stx))
     (define (obj-case->/proc stx) (make-case->/proc #t stx stx select/h))
     
-    ;; WARNING: select/h is copied from contract-arrow.ss. I'm not sure how
+    ;; WARNING: select/h is copied from contract-arrow.rkt. I'm not sure how
     ;; I can avoid this duplication -robby
     (define (select/h stx err-name ctxt-stx)
       (syntax-case stx (-> ->* ->d ->d* ->r ->pp ->pp-rest)
diff --git a/collects/mzlib/private/sigutil.rkt b/collects/mzlib/private/sigutil.rkt
index cf507a4..5119966 100644
--- a/collects/mzlib/private/sigutil.rkt
+++ b/collects/mzlib/private/sigutil.rkt
@@ -1,15 +1,15 @@
 
 (module sigutil mzscheme
-  ;; Used by unitsig.ss
+  ;; Used by unitsig.rkt
   ;; (needs an overhaul, too)
 
   (require syntax/stx
 	   syntax/struct
 	   syntax/context)
 
-  (require "sigmatch.ss")
-  (require "../unit200.ss")
-  (require "../list.ss")
+  (require "sigmatch.rkt")
+  (require "../unit200.rkt")
+  (require "../list.rkt")
 
   (define-struct signature (name      ; sym
 			    src       ; sym
diff --git a/collects/mzlib/private/stxset.rkt b/collects/mzlib/private/stxset.rkt
index b1bd074..7c056f9 100644
--- a/collects/mzlib/private/stxset.rkt
+++ b/collects/mzlib/private/stxset.rkt
@@ -5,7 +5,7 @@
 
 (provide finish-syntax-set)
 
-;; Used in the expansion of `define-syntax-set' from "etc.ss"
+;; Used in the expansion of `define-syntax-set' from "etc.rkt"
 (define-syntax (finish-syntax-set stx)
   (syntax-case stx ()
     [(_ stx)
diff --git a/collects/mzlib/runtime-path.rkt b/collects/mzlib/runtime-path.rkt
index 4fbbc81..0da6c7c 100644
--- a/collects/mzlib/runtime-path.rkt
+++ b/collects/mzlib/runtime-path.rkt
@@ -5,7 +5,7 @@
 	   setup/dirs
            scheme/list
            scheme/string
-           (only-in "private/runtime-path-table.ss" table)
+           (only-in "private/runtime-path-table.rkt" table)
            (for-syntax scheme/base))
 
   (provide define-runtime-path
diff --git a/collects/mzlib/struct.rkt b/collects/mzlib/struct.rkt
index 23a03c9..a291c64 100644
--- a/collects/mzlib/struct.rkt
+++ b/collects/mzlib/struct.rkt
@@ -6,7 +6,7 @@
 	   define-struct/properties
 	   make-->vector)
   (require-for-syntax syntax/struct
-		      "list.ss"
+		      "list.rkt"
                       syntax/stx)
 
   ;; ------------------------------------------------------------
diff --git a/collects/mzlib/thread.rkt b/collects/mzlib/thread.rkt
index 0dcf5c6..5ce3aa4 100644
--- a/collects/mzlib/thread.rkt
+++ b/collects/mzlib/thread.rkt
@@ -1,6 +1,6 @@
 
 (module thread mzscheme
-  (require "kw.ss" "contract.ss")
+  (require "kw.rkt" "contract.rkt")
 
   (provide run-server
            consumer-thread)
diff --git a/collects/mzlib/unit-exptime.rkt b/collects/mzlib/unit-exptime.rkt
index 77f1a88..e192e56 100644
--- a/collects/mzlib/unit-exptime.rkt
+++ b/collects/mzlib/unit-exptime.rkt
@@ -1,7 +1,7 @@
 #lang scheme/base
 
-(require "private/unit-syntax.ss"
-         "private/unit-compiletime.ss")
+(require "private/unit-syntax.rkt"
+         "private/unit-compiletime.rkt")
 
 (provide unit-static-signatures
          signature-members)
diff --git a/collects/mzlib/unit.rkt b/collects/mzlib/unit.rkt
index 119774a..bc71aff 100644
--- a/collects/mzlib/unit.rkt
+++ b/collects/mzlib/unit.rkt
@@ -11,18 +11,18 @@
                      scheme/struct-info
                      syntax/stx
                      unstable/location
-                     "private/unit-contract-syntax.ss"
-                     "private/unit-compiletime.ss"
-                     "private/unit-syntax.ss"))
+                     "private/unit-contract-syntax.rkt"
+                     "private/unit-compiletime.rkt"
+                     "private/unit-syntax.rkt"))
   
 (require mzlib/etc
          racket/contract/base
          scheme/stxparam
          unstable/location
-         "private/unit-contract.ss"
-         "private/unit-keywords.ss"
-         "private/unit-runtime.ss"
-         "private/unit-utils.ss"
+         "private/unit-contract.rkt"
+         "private/unit-keywords.rkt"
+         "private/unit-runtime.rkt"
+         "private/unit-utils.rkt"
          (rename-in racket/private/struct [struct struct~]))
 
 (provide define-signature-form struct struct/ctc open
diff --git a/collects/mzlib/unit200.rkt b/collects/mzlib/unit200.rkt
index c3cdbb6..68e2e60 100644
--- a/collects/mzlib/unit200.rkt
+++ b/collects/mzlib/unit200.rkt
@@ -6,8 +6,8 @@
 		      syntax/stx
 		      syntax/name
 		      syntax/context
-		      "list.ss"
-		      "private/unitidmap.ss")
+		      "list.rkt"
+		      "private/unitidmap.rkt")
 
   ;; ----------------------------------------------------------------------
   ;; Structures and helpers
diff --git a/collects/mzlib/unitsig200.rkt b/collects/mzlib/unitsig200.rkt
index 3398291..0c6256d 100644
--- a/collects/mzlib/unitsig200.rkt
+++ b/collects/mzlib/unitsig200.rkt
@@ -5,11 +5,11 @@
 ;; compile-time information). It's in dire need of an overhaul.
 
 (module unitsig200 mzscheme
-  (require "unit200.ss")
-  (require "private/sigmatch.ss")
+  (require "unit200.rkt")
+  (require "private/sigmatch.rkt")
 
-  (require-for-syntax "private/sigutil.ss")
-  (require-for-syntax "private/sigmatch.ss")
+  (require-for-syntax "private/sigutil.rkt")
+  (require-for-syntax "private/sigmatch.rkt")
   (require-for-syntax syntax/kerncase)
 
   (define-struct signed-unit (unit imports exports))
diff --git a/collects/racket/private/old-ds.rkt b/collects/racket/private/old-ds.rkt
index dec1d94..4a4e045 100644
--- a/collects/racket/private/old-ds.rkt
+++ b/collects/racket/private/old-ds.rkt
@@ -1,8 +1,8 @@
 
 (module old-ds '#%kernel
-  (#%require "define-struct.ss"
+  (#%require "define-struct.rkt"
              (for-syntax '#%kernel
-                         "stxcase-scheme.ss"))
+                         "stxcase-scheme.rkt"))
 
   (#%provide define-struct let-struct datum)