Cytochrome P4501A (CYP1A), Aroclor 1254 (A1254), and 3,3?,4,4?-tetrachlorobiphenyl (TCB) were measured in liver of winter flounder, Pleuronectes americanus, from Boston Harbor, Mass., Hempsted Harbor, N.Y., Niantic, Conn., and an offshore site, Georges Bank. We also measured CYP1A content and activity in flounder from Passamaquoddy Bay, N.B. Concentrations of A1254 and TCB were the least in fish from Georges Bank (0.46 and 0.002??g?g dry weight?1, respectively); concentrations in fish from Boston, Niantic, and Hempsted ranged from 7.6 to 11.3??g?g?1 and from 0.013 to 0.024??g?g?1. Immunodetected microsomal CYP1A contents (expressed as scup P450E equivalents) were 0.17 and 0.19?nmol?mg?1 in fish from Georges Bank and Passamaquoddy and 0.25–0.41?nmol?mg?1 in fish from Boston, Niantic, and Hempsted. Ethoxyresorufin-O-deethylase specific activities likewise were greater in fish from Boston, Niantic, and Hempsted (1.7–2.4?nmol?min?1?mg?1) than in fish from Georges Bank or Passamaquoddy (0.83 and 0.61?nmol?min?1?mg?1). CYP1A content and activity were correlated with hepatic concentrations of A1254 and TCB. These data, together with data reported in previous studies, indicate that strong induction of CYP1A protein occurs in winter flounder populations along most of the industrialized east coast and that induction of CYP1A is common, but less strong, at sites distant from the urban centers of the Northeast.