Showing or hiding from usergroups

[mrtroybaker]

I have this code:
<if condition="is_member_of($bbuserinfo,6,7,2,5,16,1,3 )"><!-- RevSense Ad Code -->
<script src="http://www.myreptileplanet.com/revsense/index.php?section=serve&id=36&output=js"></script></if>

However usergroup 18 can still see it. Is the cose right? Whats the deal? This is in the header btw. Thanks for the help!