diff --git a/lib/prime.rb b/lib/prime.rb
index ebe012b..9eb66de 100644
--- a/lib/prime.rb
+++ b/lib/prime.rb
@@ -99,9 +99,24 @@ class Prime
     warn "Prime::new is obsolete. use Prime::instance or class methods of Prime."
   end
 
+  def include?(n) # :nodoc:
+    prime?(n) # avoid infinite loop
+  end
+
   class << self
     extend Forwardable
     include Enumerable
+
+    def include?(n) # :nodoc:
+      if n.is_a?(Module)
+        # Class#include?
+        included_modules.include?(n)
+      else
+        # Enumerable#include?
+        prime?(n) # avoid infinite loop
+      end
+    end
+
     # Returns the default instance of Prime.
     def instance; @the_instance end
 
diff --git a/test/test_prime.rb b/test/test_prime.rb
index 885406f..01e5ee8 100644
--- a/test/test_prime.rb
+++ b/test/test_prime.rb
@@ -171,4 +171,16 @@ class TestPrime < Test::Unit::TestCase
 
     assert_not_include Prime.each(7*37).to_a, 7*37, "[ruby-dev:39465]"
   end
+
+  def test_include?
+    assert Prime.include?(2)
+    assert !Prime.include?(4)
+
+    assert Prime.instance.include?(2)
+    assert !Prime.instance.include?(4)
+
+    # Class#include?
+    assert Prime.include?(Enumerable)
+    assert !Prime.include?(String)
+  end
 end