1. Nonylphenol is an environmental estrogenic compound. Little is known about its interaction with aquatic species since most of the work on oestrogenic alkylphenols has been done in vitro using cells in culture. 2. Rainbow trout (Oncorhynchus mykiss) were exposed to 14C-nonylphenol at 18 and 36 ppb in water to study its distribution, persistence, and bioaccumulation. 3. Nonylphenol, or its metabolites, were distributed through the body of rainbow trout including the edible tissues of dorsal muscle and fat. 4. Nonylphenol was rapidly taken up into most tissues and had an apparent half-life of 19-20 +/- 4 SE hours in the muscle and fat. 5. The apparent bioaccumulation factor in viscera and carcass ranged from 40 in carcass to 100 in viscera. 6. Three glucuronide metabolites were separated by thin-layer chromatography following treatment of bile with beta-glucuronidase.