Shoaling behaviour provides antipredator benefits that rely, to some extent, on a high degree of phenotypic homogeneity between individuals within the shoal. Therefore, fish should have the ability to discriminate between potential shoalmates, choosing to associate with individuals of similar appearance to themselves. We studied the effects of a single phenotypic character, body coloration, on association choices made by black and white mollies (Poecilia latipinna). When given a choice between a group of mollies of similar coloration and an empty compartment, individual test fish (black or white) spent significantly more time near the fish group. When given a choice between a group of black mollies and a group of white mollies, individual fish (black or white) spent significantly more time near the group of mollies of similar coloration to their own. When given a choice between a group of mollies of dissimilar coloration and an empty compartment, black and white mollies reacted differently. Black mollies spent significantly more time on the side of the central compartment closest to the white mollies, while there was no significant difference between the time spent by white mollies on either side of the test tank. Our results indicate that fish can use visual cues to discriminate actively between potential shoalmates on the basis of body coloration. Copyright 1998 The Association for the Study of Animal Behaviour.