Index: ThreadOps.cs
===================================================================
--- ThreadOps.cs	(revision 93)
+++ ThreadOps.cs	(working copy)
@@ -64,6 +64,17 @@
 //    abort_on_exception=
 //    critical
 //    critical=
+
+        [RubyMethod("critical=", RubyMethodAttributes.PublicSingleton)]
+        public static bool SetCritical(CodeContext/*!*/ context, object self, bool critical) {
+            if (critical)
+                Thread.BeginCriticalRegion();
+            else
+                Thread.EndCriticalRegion();
+
+            return true;
+        }
+
 //    current
 //    exclusive
 //    exit