When pairs of convict cichlids (Cichlasoma nigrofasciatum) were forced to compete for breeding sites, intruding pairs were not able to displace residents when the intruding pair was the same size as the resident pair. This prior resident asymmetry was over-ridden when each individual of the intruding pair was larger than its same-sex counterpart in the resident pair. When only one member of the resident pair (of either sex) was the same size as its same-sexed counterpart while its mate was smaller than its counterpart in the intruding pair, the residents were able to retain control of the breeding site. Thus, only one member of the resident pair needed to be similar in size to its same-sex counterpart for the residents to retain control. Single residents, of either sex, were either displaced by the intruding pair, or formed a pair with an intruder and then remained on the site.