In an ongoing experiment, we investigated how mating behaviors of male western mosquitofish Gambusia affinis change across a range of temperatures. During behavioral trials, we observed high levels of homosexual behavior between males when in a competitive situation. We observed that when two males are in competition for matings with females, the smaller male performed significantly more homosexual acts than the larger. These results suggest that a dominance relationship develops with males in competition, where large males may dominate small males by not allowing them to copulate with the female, thereby causing an increase in same-sex mating by smaller males. Although preliminary data also indicate a possible relationship between density of males and number of homosexual copulations, we were not able to statistically analyze the relationship because these observations occurred during an ongoing experiment. In a future study, we will seek to understand underlying causes of homosexual behavior in the western mosquitofish.