Or do you mean more than one athlete with an affected foot? That would be athletes' foot.
Actually, that would probably be "athletes' feet" unless there was only a single infected foot among them... but in that case, it would simply be "athlete's foot" because you couldn't rightly say that the infected foot belonged to more than one athlete...
Of course, the answer is "athlete's foot" anyway. :-)
-sauoq "My two cents aren't worth a dime.";