Polymorphism in male morphology is often correlated with the expression of alternate behavioral tactics. This relationship between behavioral and morphological polymorphisms, however, is less well understood. We characterized male mating behaviors and morphological variation within and between Poecilia velifera and P. petenensis to understand mating signal evolution in the sailfin molly lineage. In addition, we examined whether differences between these species in the size range of mature males and the strength of allometry between dorsal fin size and body length could explain the variation observed in their expression of different mating behaviors. We determined each male's mating behavior profile by observing the behavior of a single male in the presence of a receptive female. We found that P. velifera showed evidence of an alternate male mating strategy, with small males generally performing only gonopodial thrusts (forced insemination attempts) towards receptive females, while large males performed courtship displays as well as gonopodial thrusts. Males of P. petenensis performed similar rates of courtship displays and gonopodial thrusts regardless of body length. Little variation existed among populations of P. velifera in mating behaviors, while males from different populations of P. petenensis showed population-specific average rates of each mating behavior. Population differences in P. petenensis may be driven, in part, by its occurrence in more variable habitats than those occupied by P. velifera. Variation among individuals in the mating repertoire of P. velifera, but not P. petenensis, suggests that the greater range of variation in male size at maturity, as well as considerably stronger allometry between dorsal fin size and body length, may explain why males of P. velifera show the greatest degree of expression of alternate male mating behaviors when compared to other sailfin species. These results also suggest an important role of morphological polymorphisms in predicting the expression of alternate male mating behaviors.