diff --git a/js-assembler/runtime-src/baselib-lists.js b/js-assembler/runtime-src/baselib-lists.js
index a4ad775..b892267 100644
--- a/js-assembler/runtime-src/baselib-lists.js
+++ b/js-assembler/runtime-src/baselib-lists.js
@@ -172,7 +172,7 @@
 
 
     // Coerse a list back into a JavaScript array.
-    var listToArray = function(lst) {
+    var listToArray = function (lst) {
         var result = [];
         while (lst !== EMPTY) {
             result.push(lst.first);
@@ -185,12 +185,15 @@
     // isList: Any -> Boolean
     // Returns true if x is a list (a chain of pairs terminated by EMPTY).
     var isList = function (x) { 
-        while (x !== EMPTY) {
-            if (x instanceof Cons) {
-                x = x.rest;
-            } else {
-                return false;
-            }
+        var tortoise, hare;
+        tortoise = hare = x;
+        while (true) {
+            if (tortoise instanceof Cons) { tortoise = tortoise.rest; }
+            if (hare instanceof Cons) { hare = hare.rest; }
+            if (hare instanceof Cons) { hare = hare.rest; }
+            if (hare === EMPTY) { return true; }
+            if (tortoise === hare) { return false; }
+            if ((hare instanceof Cons) === false) { return false; }
         }
         return true;
     };
diff --git a/tests/more-tests/lists.expected b/tests/more-tests/lists.expected
index c859ffb..3d52e80 100644
--- a/tests/more-tests/lists.expected
+++ b/tests/more-tests/lists.expected
@@ -2,6 +2,10 @@
 ("hello" "world")
 true
 true
+true
+false
+true
+true
 false
 true
 hello
diff --git a/tests/more-tests/lists.rkt b/tests/more-tests/lists.rkt
index bc49d82..703d4d5 100644
--- a/tests/more-tests/lists.rkt
+++ b/tests/more-tests/lists.rkt
@@ -3,6 +3,11 @@
 '(1 2 3)
 (list "hello" "world")
 
+(list? empty)
+(list? '(1))
+(list? '(1 2))
+(list? 1)
+
 (empty? empty)
 (empty? '())
 (cons? '())
diff --git a/tests/more-tests/sharing.expected b/tests/more-tests/sharing.expected
index 8b7c869..bd6f4c7 100644
--- a/tests/more-tests/sharing.expected
+++ b/tests/more-tests/sharing.expected
@@ -22,3 +22,5 @@ jill
 jane
 
 
+true
+false
diff --git a/tests/more-tests/sharing.rkt b/tests/more-tests/sharing.rkt
index 55ebb7a..3a95aa6 100644
--- a/tests/more-tests/sharing.rkt
+++ b/tests/more-tests/sharing.rkt
@@ -70,4 +70,12 @@
   (for-each displayln (map person-name (person-friends b)))
   (newline)
   (for-each displayln (map person-name (person-friends c)))
-  (newline))
\ No newline at end of file
+  (newline))
+
+
+
+;; Make sure cyclic lists are treated correctly by list?
+(shared ([a (cons 1 a)])
+  (begin
+    (displayln (pair? a))
+    (displayln (list? a))))
\ No newline at end of file