The livebearing fishes of the genus Gambusia are quickly becoming a model system to test alternative mechanisms of sexual selection in the form of coercive mating. I investigated effects of male and female body size, and correlated characteristics on male mating behavior in the western mosquitofish, G. affinis. Because larger females typically have larger broods in Gambusia, I predicted that males would attempt more copulations with larger females. Two-way ANOVA showed that female body size was a significant predictor of male mating behavior but male size was not. I also tested the effects of a suite of additional traits (both male and female) on male mating attempts. In a stepwise multiple regression, female size (SL), size of the gravid spot, and male testes mass were significant predictors of male mating attempts, accounting for about 27% of variation in male mating. Path analysis showed that differences between male and female body size, male body condition, and male testes mass were significant predictors of male mating attempts, and also accounted for 27% of the variation in male mating attempts. The two statistical models were very similar in their predictive power, but differed slightly in significant predictor variables. My results confirm that factors other than female size are important predictors of male mating behavior in the western mosquitofish.