Displaying results from a query properly.

Hi there. This isn't a major problem, more of a matter of aesthetics. Basically, I have a query which pulls a list of usernames from the database (based on their user level). These users should then listed in this fashion. Administrators: allydm, alex, mike, dave Moderators: big 'd', frank, william However, the loop that I have returns the results like this... Administrators: allydm, alex, mike, dave, Moderators: big 'd', frank, william, There is a , after each username, including the last one in that list. I'm after a way of making the usernames display with a , after them, apart from the last username in that set of results. My current display code is: <?php do {echo ''.$administrators['username'].', ';} while ($administrators = mysql_fetch_assoc($administrators_result)) ?> How could I change to display the results in the fashion that I'd like? Many thanks for any suggestions.