diff --git a/actionpack/test/dispatch/mime_type_test.rb b/actionpack/test/dispatch/mime_type_test.rb
index <HASH>..<HASH> 100644
--- a/actionpack/test/dispatch/mime_type_test.rb
+++ b/actionpack/test/dispatch/mime_type_test.rb
@@ -18,7 +18,7 @@ class MimeTypeTest < ActiveSupport::TestCase
       assert_equal Mime::Type[:MOBILE], Mime::EXTENSION_LOOKUP['mobile']
 
       Mime::Type.unregister(:mobile)
-      assert !Mime.const_defined?(:MOBILE), "Mime::MOBILE should not be defined"
+      assert !Mime::Type.registered?(:MOBILE), "Mime::MOBILE should not be defined"
       assert !Mime::LOOKUP.has_key?('text/x-mobile'), "Mime::LOOKUP should not have key ['text/x-mobile]"
       assert !Mime::EXTENSION_LOOKUP.has_key?('mobile'), "Mime::EXTENSION_LOOKUP should not have key ['mobile]"
     ensure
@@ -183,6 +183,10 @@ class MimeTypeTest < ActiveSupport::TestCase
     end
   end
 
+  test "deprecated const_defined?" do
+    assert_deprecated { Mime.const_defined?(:ALL) }
+  end
+
   test "verifiable mime types" do
     all_types = Mime::SET.symbols
     all_types.uniq!
Added test case for deprecation of Mime.const_defined? and removed use of it.