Male alternative reproductive strategies are found in some species of most major animal taxa, but are especially widespread in fishes. Mature males of the shortfin molly, Poecilia gillii, display extensive variation in size and morphology. We devised a field test of a priori hypotheses regarding the interrelationships between male size, colouration, morphology, and mating tactics. Males did not occur in discrete size classes, but instead occurred in a size and morphological continuum. Large males exhibited darker and more orange coloured dorsal and caudal fins, while small males exhibited lighter and more inconspicuous fin colouration. Furthermore, larger males had proportionately deeper bodies, larger dorsal and caudal fins, and shorter gonopodia than smaller males. Our field study of male mating behaviour revealed a lack of courtship in this species, and similar levels of mating attempts (gonopodial thrusts) irrespective of male size. Instead, small males were significantly more likely to chase females than were large males. In contrast, large males exhibited higher rates of gonoporal nibbling (a likely means by which males determine, through chemical factors, whether a female is carrying fertilizable ova) and higher likelihood of chasing other males away. In total, we found evidence for the predicted associations between male size, colouration, morphology and mating behaviour – associations that appear likely to maximize mating success for males of a given body size and phenotype.