using jquery ajax with codeigniter

[eluser]fatfishy[/eluser]
I tried to post the full url, as you'r code does .. but I still get
-- Status Code:500 Internal Server Error --

but when I manually get into - http://127.0.0.1/index.php/ajax/username_taken
it works .. (gives an error, Message: Undefined index: username Filename: controllers/ajax.php which is ok ...) and printing "not exists" which is also ok,

what can cause the problem ? why the ajax cant rich the controller ?

thanks!