Elaborateness of precopulatory mating behavior in the members of 13 genera of poecili-id fishes is found related to the length of the external male genitalium (modified anal fin or gonopodium) and the presence or absence of specializations of the paired fins that assist gonopodial action. It is suggested that the underlying difference between the sexual behaviors of species with long and short gonopodia involves the differential utilization of visual, tactile and kinesthetic stimuli necessary for the successful transfer of sperms. Males of species with long gonopodia may be primarily visually oriented and may therefore be able to contact the female while she is in motion. Males of species with shorter gonopodia may rely more heavily on kinesthetic and tactile cues; copulatory contacts are typically made while the female is stationary. In the latter forms, elaborate display and occasional male secondary sexual coloration may evoke in the female a response to slow down or stop. Whereas elaborate preparatory display, color dimorphism and modifications of the paired fins have evolved so that they enhance the effectiveness of the short type of gonopodium, permanent structural asymmetry may have arisen for the same reason in the long gonopodium. A profound influence of asymmetry of the gonopodial system on certain behavioral items is demonstrated.