[support] trying to create zebra striping in custom block

I have a custom block showing the users whose content has attracted most pageviews. part of my code is this: print "<tr class=\""; print $zebra; print "\"><td>"; this is a drupal 6 installation and I don't seem to be able to get the odd/even classes I'm looking for. If I put print "odd", they all come out grey, "even" and they all come out white, so I know the syntax of the code is fine...I just can't get the zebra variable working. any tips what I'm failing to do?? Neil