Female convict cichlids, Cichlasoma nigrofasciatum, typically develop orange ventral coloration which males do not. We investigated the behavioural significance of this sexual dichromatism using both live and dummy stimulus fish. In our experiments, males were insensitive to variation in ventral coloration. Females, however, appeared to use this colour pattern as a visual cue. Females directed significantly more behaviour, including aggressive behaviour, towards more brightly coloured stimulus fish, as well as towards dummies with orange ventral coloration compared with those without. Bright females displayed the least aggression towards an intruding female of intermediate coloration. Thus, sexual dichromatism in the convict cichlid is shown to affect intrasexual aggression among females. Copyright 1998 The Association for the Study of Animal Behaviour.