Sperm motility, density, adenosine triphosphate (ATP) concentration, and fertilization rates of sex-reversed female (XX-male) rainbow trout (Oncorhynchus mykiss) were compared to those of normal (XY-male) males. The relationships between sperm traits, fertilization rates, and characteristics of individual fish (weight, gonadosomatic index) were assessed. The fertilization rate of sperm stripped from normal males averaged 81% (s.d. 16%). The sperm of sex-reversed females had to be removed directly from the testes and achieved average fertilization rates of 60% (s.d. 13%). Testicular sperm from normal males was motile for shorter periods of time and produced lower fertilization rates than testicular sperm from sex-reversed fish. Fertilization rates were positively correlated with higher initial sperm motility, but not with sperm density or ATP concentrations.