Males of two swordtail colour breeds (Xiphophorus) were tested for differences related to aggression in a prior residency situation. The median latency time of the first dominance sign, the first attack, and the first approach was significantly shorter in the red than in the black breed, while these differences were not present when resident and newcomer or the larger and the smaller member of the pair were compared, either in the two breeds separately, or in the two breeds combined. The significance of these differences in agonistic behaviour is discussed in relation to resource holding power and colour variants in fish.