diff --git a/lib/rdoc/options.rb b/lib/rdoc/options.rb
index 1d92bd4748..ee878caa7e 100644
--- a/lib/rdoc/options.rb
+++ b/lib/rdoc/options.rb
@@ -327,8 +327,10 @@ def parse(argv)
 
       opt.separator nil
 
+      generator_text = @generators.keys.map { |name| "  #{name}" }.sort
+
       opt.on("--fmt=FORMAT", "--format=FORMAT", "-f", @generators.keys,
-             "Set the output formatter.") do |value|
+             "Set the output formatter.  One of:", *generator_text) do |value|
         @generator_name = value.downcase
         setup_generator
       end