Male sailfin mollies, Poecilia latipinna, exhibit alternative mating strategies that are correlated with body size. Larger males court females and often prevent smaller males from accessing females, while smaller males often forcefully inseminate females. This pattern is well documented in the laboratory but has not been studied thoroughly in the field, where patterns of mating behavior are known to vary among natural populations. In an observational study of a P. latipinna population, we measured the proportion of time males from three size classes (small, intermediate, and large) spent exhibiting five mating behaviors: courting, thrusting, nibbling, chasing other males away, and being chased. We predicted that large males would spend more time courting females and chasing away other males, while small males would spend more time thrusting, nibbling, and being chased by other males. We also predicted that intermediate males would behave intermediately. Our findings partially supported those of previous laboratory studies: small males spent more time attempting to forcefully inseminate females but large males did not spend more time courting. Courtship displays were rare in our observations. Large males did spend more time chasing away other males however. Intermediate males behaved intermediately in some cases but also exhibited mating behavior typically characteristic of large males.