diff --git a/include/cyclone/runtime.h b/include/cyclone/runtime.h
index 69492d23..8fe19f8f 100644
--- a/include/cyclone/runtime.h
+++ b/include/cyclone/runtime.h
@@ -235,6 +235,7 @@ object Cyc_is_procedure(void *data, object o);
 object Cyc_is_macro(object o);
 object Cyc_is_eof_object(object o);
 object Cyc_is_cvar(object o);
+object Cyc_is_opaque(object o);
 object Cyc_sum_op(void *data, common_type * x, object y);
 object Cyc_sub_op(void *data, common_type * x, object y);
 object Cyc_mul_op(void *data, common_type * x, object y);
diff --git a/runtime.c b/runtime.c
index 84c7f463..ae1916b6 100644
--- a/runtime.c
+++ b/runtime.c
@@ -1115,6 +1115,13 @@ object Cyc_is_cvar(object o)
   return boolean_f;
 }
 
+object Cyc_is_opaque(object o)
+{
+  if ((o != NULL) && !is_value_type(o) && ((list) o)->tag == c_opaque_tag)
+    return boolean_t;
+  return boolean_f;
+}
+
 object Cyc_eq(object x, object y)
 {
   if (x == y)