Topminnows of the genus Poeciliopsis are susceptible to hepatocarcinogenesis by waterborne exposure to procarcinogenic polycyclic aromatic hydrocarbons (PAH). We examined induction of cytochrome P4501A (CYP1A) in liver and other organs of the species P. monacha and P. lucida exposed to benzo[a]pyrene (B[a]P) in water (added in acetone carrier) at 1 mg/l for 48 and 90 h. Fish were fixed whole in formalin, and CYP1A was examined immunohistochemically in sagittal sections of whole animals by staining with monoclonal antibody 1-12-3, which recognizes a single cross-reacting CYP1A protein in Poeciliopsis liver microsomes. Fish exposed to B[a]P for 48 h showed moderate staining, and those exposed for 90 h showed strong specific staining in various epithelial cells in both species. These included hepatocytes, pancreatic cells, epithelial cells in gill, enterocytes of the gut, and kidney tubular epithelium. Endothelial cells in several organs, including gill pillar cells and endocardial cells in the heart, showed strong staining. Staining was stronger in P. monacha than in P. lucida. Untreated animals of both species showed mild staining of the same cells stained in B[a]P-treated fish. In P. monacha, carrier (acetone) elicited a moderate increase in staining in most cell types, including those of liver and gill; the basis for this acetone effect is not known. There was a very strong specific induction by B[a]P in olfactory epithelium and epidermal taste bud epithelium of P. monacha, the first demonstration of strong CYP1A induction in chemosensory epithelia exposed to inducer in a physiologically relevant way. This study clearly establishes that waterborne PAH can elicit induction of P4501A proteins in multiple cell types in many organs of fish, with some sites of induction (olfactory epithelium) possibly related directly to the route of exposure. The species differences in the induction response, with induction in liver and some other organs generally being greater in P. monacha than in P. lucida, could be related to previously recognized species differences in PAH toxicities in Poeciliopsis.