[themes] passing variables to a tlp.php file

Hi, this is probably a basic question for most, but i am stuck on this. I have a custom module enabled which has a function: theme_mymod_myfcn($myarg) { ... } I am trying to override this fcn via the file: mymod_myfcn.tpl.php However, whenever i try to print $myarg in the tpl.php file it appears to be empty. Why? Is there a way to access the $myarg variable in the tpl.php file. thanks matt