Fluoxetine is a biologically active pharmaceutical chemical that has been detected at parts-per-trillion levels in surface waters in North America and Europe. This has generated concern because negative effects in aquatic organisms are possible. Known as a selective serotonin reuptake inhibitor, fluoxetine (e.g., Prozac; Elli Lilly) is neurologically active and widely prescribed for clinical depression in humans. In the present investigation, acute and chronic toxicities of fluoxetine were evaluated in an environmentally relevant species, western mosquitofish Gambusia affinis. Acute toxicity (5 to 5340 ppb fluoxetine) was assessed in neonates (age 24 to 48 hours) exposed in glass aquaria for 7 days; chronic toxicity (0.05 to 5 ppb fluoxetine) was examined in fish exposed from age neonate to age 91 days; and effects of chronic exposure (100 days) on sexual maturation was investigated in mesocosm tanks (100 L) in fish exposed (7 to 71 ppb) from age 59 to 159 days. Acute toxicity of fluoxetine in neonate western mosquitofish was observed to have a 7-day median lethal concentration of 546 ppb. Chronic exposure did not affect survival, growth, or sex ratio; however, increased lethargy in fish exposed to > or =0.5 ppb fluoxetine was observed. In fish exposed from age 59 to 159 days (juvenile to adult life stages), delayed development of external adult sexual morphology was observed at 71 ppb fluoxetine, which consisted of delayed onset of the presence of the black spot in the posterior abdomen in female fish and delayed formation of the elongated anal fin (gonopodium) in male fish. The present study demonstrated that chronic exposure of western mosquitofish to fluoxetine can affect sexual development; however, it does so only at concentrations 3 to 4 orders of magnitude higher than those previously found in the environment.