The effects of male and female body size, and correlated characteristics, on male mating behaviour were investigated in the western mosquitofish Gambusia affinis. Because larger females typically have larger broods in Gambusia sp., it was 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 behaviour but male size was not. The effects of a suite of additional traits (both male and female) on male mating attempts were also tested. In a stepwise multiple regression, female standard length (LS), size of the female gravid spot and male testes mass were significant predictors of male mating attempts, accounting for c. 27% of variation in male mating. Path analysis showed that differences between male and female LS, 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. Results confirm that factors other than female size are important predictors of male mating behaviour in the western mosquitofish.