diff --git a/raco.rkt b/raco.rkt
index eb1bc2f..67dd993 100644
--- a/raco.rkt
+++ b/raco.rkt
@@ -8,9 +8,9 @@
   (define coverage-dir "coverage")
   (define output-format "html")
   (define exclude-dirs '())
+  (define include-exts '())
 
-  (define files
-    (expand-directories
+  (define args
      (command-line
       #:program (short-program+command-name)
       #:once-each
@@ -27,8 +27,12 @@
       [("-e" "--exclude-from-output") t
        "exclude all directories named this from the coverage report. By default excludes dirs named tests"
        (set! exclude-dirs (cons t exclude-dirs))]
+      [("-i" "--include-extentions") f
+       "include these extentions in files to cover."
+       (set! include-exts (cons f include-exts))]
       #:args (file . files)
-      (cons file files))))
+      (cons file files)))
+  (define files (expand-directories args include-exts))
   (define generate-coverage
     (case output-format
       [("html") generate-html-coverage]
@@ -47,7 +51,8 @@
 
 ;; TODO allow for arbitrary extensions
 (define extensions '(#rx"\\.rkt$" #rx"\\.ss$"))
-(define (expand-directories files)
+(define (expand-directories files [exts null])
+  (define comped (map regexp exts))
   (flatten
    (for/list ([f files])
      (if (not (directory-exists? f))
@@ -56,22 +61,22 @@
                          (if (absolute-path? f)
                              f
                              (build-path (current-directory) f))])
-           (expand-directory))))))
+           (expand-directory (append extensions comped)))))))
 
 ;; -> (HorribyNestedListsOf PathString)
-(define (expand-directory)
+(define (expand-directory exts)
   (for/list ([p (directory-list)])
     (cond [(directory-exists? p)
            (parameterize ([current-directory (build-path (current-directory) p)])
-             (expand-directory))]
-          [(ormap (lambda (r) (regexp-match r (path->string p))) extensions)
+             (expand-directory exts))]
+          [(ormap (lambda (r) (regexp-match r (path->string p))) exts)
            (path->string (build-path (current-directory) p))]
           [else null])))
 (module+ test
   (define-runtime-path cur ".")
   (parameterize ([current-directory (build-path cur "tests/basic")])
     (check-equal? (list->set (map (compose path->string ->relative)
-                                  (flatten (expand-directory))))
+                                  (flatten (expand-directory extensions))))
                   (set "prog.rkt"
                        "not-run.rkt"))))
 
diff --git a/scribblings/basics.scrbl b/scribblings/basics.scrbl
index b07afa2..7a20b82 100644
--- a/scribblings/basics.scrbl
+++ b/scribblings/basics.scrbl
@@ -27,5 +27,8 @@ The @exec{raco cover} command accepts the following flags:
                --- excludes any directories by given name from the coverage report.
                Files in these directories are still run, they are just excluded from the
                outputted coverage. This flag may appear any number of times.}
+         @item{@Flag{i} or @DFlag{include-extentions}
+               --- include the given regular expression in the list of file patterns
+                   used when expanding directories, searching for files to cover.}
          @item{@Flag{v} or @DFlag{verbose}
                --- enable verbose logging}]