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Life history traits such as time to and size at maturity are influenced by both genetic and environmental factors. One such environmental factor may be the social environment in which a juvenile is reared. This study examined the life history traits of sailfin mollies (Poecilia latipinna) that were reared in varying social environments: in isolation, in groups with an adult male present, and in groups without an adult male present. Since females prefer larger males, we anticipated that males reared in the presence of an adult male should delay maturation and therefore mature at larger sizes in order to better compete for mates. We found that males reared with an adult male took three times longer to mature and were almost 30 percent larger than males reared without adult males or in isolation. Our results suggest that social environment has significant effects on the final size at maturity, and therefore fitness, of male sailfin mollies.