The reproductive performances of growth-enhanced transgenic, hatchery, and cultured nontransgenic coho salmon Oncorhynchus kisutch were examined to investigate the consequences of reproductive interaction between growth hormone (GH)?transgenic fish and wild fish that may occur if transgenic salmon escaped into the natural environment. We examined adult morphological phenotypes, gamete quantity and quality, in vitro offspring production, courtship and spawning behavior, male competitive behavior, and transgene transmission to offspring. Transgenic, hatchery, and cultured nontransgenic fish required 2, 3, and 3 or 4 years, respectively, to reach sexual maturation. No differences in male gamete quantity or in vitro offspring production were observed. Transgenic females were more fecund than hatchery females but had smaller eggs. Fewer transgenic females spawned than hatchery females under experimental conditions, and transgenic females displayed consistently low levels of courtship behavior. In noncompetitive trials, there were no differences in the courtship behavior of transgenic and hatchery males; during competition with hatchery males, however, transgenic males failed to spawn and displayed less courtship and competitive behavior. Cultured nontransgenic salmon also displayed reduced spawning capacity relative to hatchery salmon, indicating that the effects observed in transgenic salmon may arise in part from being reared in the culture environment and highlighting the difficulty in using laboratory-reared transgenic fish to assess reproductive fitness because of the strong genotype?environment interactions. As long as wild-reared transgenic fish are unavailable, exact determinations of reproductive fitness will be difficult. However, these studies have shown that in a simulated natural environment, growth-enhanced transgenic coho salmon do display courtship behavior and can spawn, producing viable transgenic offspring. The findings suggest some capacity exists for the natural transmission of transgenes to populations arising from reproductive interaction, which could occur during first contact between escaped cultured transgenic fish and wild conspecifics.