Trade offs between mating and predator avoidance advantages were studied in the western mosquitofish (Gambusia affinis: Poeciliidae). When tested with green sunfish (Lepomis cyanellus) predators, larger male G. affinis were found to have a higher number of escapes compared to smaller males. Smaller males were captured more often than larger males in groups of six and in both complex and simple habitat types. Large males, on average, were found to attempt more matings and spend a larger proportion of time near the female than smaller males. When trials with predators were compared to trials without predators, large males decreased mating attempts and time spent near the female although, small males increased time spent near females. Size of the predator did not affect these behaviors. Fry were reared under varying levels of predator exposure (visual, olfactory, both, or neither) and predation response was measured for each group at maturity. Results were not significant and were confounded by G. affinis size.