Triclosan (TCS) is an antibacterial agent used in a variety of personal care and industrial products. Triclosan and its environmentally transformed derivative, methyl-TCS, have been detected in waters receiving effluent from public wastewater treatment plants. Previous studies have demonstrated that TCS has the potential to act as an endocrine disruptor. The present study tested the hypothesis that TCS acts as an endocrine-disrupting agent in fish. Mature male western mosquitofish, Gambusia affinis, were exposed to TCS concentrations of 100, 200, and 350nM (29.0, 57.9, and 101.3 μg/L) for 35 d by the static renewal method. Induction of the normally female-limited vitellogenin gene expression and reduction in sperm count were quantified as biomarkers of endocrine disruption. Vitellogenin mRNA expression was significantly elevated in the 350nM TCS treatment. Sperm counts in the same treatment group were significantly decreased. The mean hepatosomatic index in the 350nM treatment group was significantly increased. This study demonstrates that TCS has the potential to act as an endocrine disruptor in male mosquitofish. Environ. Toxicol. Chem. 2010;29:1287-1291. © 2010 SETAC.