diff --git a/src/org/mozilla/javascript/NativeMath.java b/src/org/mozilla/javascript/NativeMath.java
index 61c495ad4f..dc8ab9b7c2 100644
--- a/src/org/mozilla/javascript/NativeMath.java
+++ b/src/org/mozilla/javascript/NativeMath.java
@@ -185,7 +185,33 @@ private static Object clz32(Context cx, Scriptable scope, Scriptable thisObj, Ob
         if (n == 0) {
             return Double32;
         }
-        return Double.valueOf(31 - Math.floor(Math.log(n >>> 0) * LOG2E));
+
+        int place = 0;
+        if ((n & 0xFFFF0000) != 0) {
+            place += 16;
+            n >>>= 16;
+        }
+        if ((n & 0xFF00) != 0) {
+            place += 8;
+            n >>>= 8;
+        }
+        if ((n & 0xF0) != 0) {
+            place += 4;
+            n >>>= 4;
+        }
+        if ((n & 0b1100) != 0) {
+            place += 2;
+            n >>>= 2;
+        }
+        if ((n & 0b10) != 0) {
+            place += 1;
+            n >>>= 1;
+        }
+        if ((n & 0b1) != 0) {
+            place += 1;
+        }
+
+        return Double.valueOf(32 - place);
     }
 
     private static Object cos(Context cx, Scriptable scope, Scriptable thisObj, Object[] args) {
diff --git a/testsrc/jstests/harmony/math-clz32.js b/testsrc/jstests/harmony/math-clz32.js
new file mode 100644
index 0000000000..8b9d60bb5a
--- /dev/null
+++ b/testsrc/jstests/harmony/math-clz32.js
@@ -0,0 +1,54 @@
+load("testsrc/assert.js");
+
+assertEquals(Math.clz32(64), 25);
+assertEquals(Math.clz32(0), 32);
+
+assertEquals(Math.clz32(0x00000001), 31);
+assertEquals(Math.clz32(0x00000002), 30);
+assertEquals(Math.clz32(0x00000004), 29);
+assertEquals(Math.clz32(0x00000008), 28);
+assertEquals(Math.clz32(0x000000010), 27);
+assertEquals(Math.clz32(0x00000020), 26);
+assertEquals(Math.clz32(0x00000040), 25);
+assertEquals(Math.clz32(0x00000080), 24);
+assertEquals(Math.clz32(0x00000100), 23);
+assertEquals(Math.clz32(0x00000200), 22);
+assertEquals(Math.clz32(0x00000400), 21);
+assertEquals(Math.clz32(0x00000800), 20);
+assertEquals(Math.clz32(0x00001000), 19);
+assertEquals(Math.clz32(0x00002000), 18);
+assertEquals(Math.clz32(0x00004000), 17);
+assertEquals(Math.clz32(0x00008000), 16);
+assertEquals(Math.clz32(0x00010000), 15);
+assertEquals(Math.clz32(0x00020000), 14);
+assertEquals(Math.clz32(0x00040000), 13);
+assertEquals(Math.clz32(0x00080000), 12);
+assertEquals(Math.clz32(0x00100000), 11);
+assertEquals(Math.clz32(0x00200000), 10);
+assertEquals(Math.clz32(0x00400000), 9);
+assertEquals(Math.clz32(0x00800000), 8);
+assertEquals(Math.clz32(0x01000000), 7);
+assertEquals(Math.clz32(0x02000000), 6);
+assertEquals(Math.clz32(0x04000000), 5);
+assertEquals(Math.clz32(0x08000000), 4);
+assertEquals(Math.clz32(0x10000000), 3);
+assertEquals(Math.clz32(0x20000000), 2);
+assertEquals(Math.clz32(0x40000000), 1);
+assertEquals(Math.clz32(0x80000000), 0);
+assertEquals(Math.clz32(0xFFFFFFFF), 0);
+assertEquals(Math.clz32(0xFFFF0000), 0);
+assertEquals(Math.clz32(0x0000FF00), 16);
+assertEquals(Math.clz32(0x000000F0), 24);
+
+assertEquals(Math.clz32(-0), 32);
+assertEquals(Math.clz32(-1), 0);
+assertEquals(Math.clz32(-100), 0);
+
+assertEquals(Math.clz32(1.9), 31);
+assertEquals(Math.clz32(Number.POSITIVE_INFINITY), 32);
+assertEquals(Math.clz32(Number.NEGATIVE_INFINITY), 32);
+assertEquals(Math.clz32(Number.NaN), 32);
+
+assertEquals(Math.clz32(0x1_0000_0100), 23);
+
+"success";
diff --git a/testsrc/org/mozilla/javascript/tests/harmony/MathClz32Test.java b/testsrc/org/mozilla/javascript/tests/harmony/MathClz32Test.java
new file mode 100644
index 0000000000..47632d4cf6
--- /dev/null
+++ b/testsrc/org/mozilla/javascript/tests/harmony/MathClz32Test.java
@@ -0,0 +1,7 @@
+package org.mozilla.javascript.tests.harmony;
+
+import org.mozilla.javascript.drivers.RhinoTest;
+import org.mozilla.javascript.drivers.ScriptTestsBase;
+
+@RhinoTest(value = "testsrc/jstests/harmony/math-clz32.js")
+public class MathClz32Test extends ScriptTestsBase {}