Many fish treated with polynuclear aromatic hydrocarbons (PAH) or certain other environmental chemicals respond with an apparent induction of form(s) of hepatic cytochrome P-450 that catalyze monooxygenase reactions having characteristics distinct from those seen in control animals. However, hepatic monooxygenase systems in some species seem to possess properties like those induced by PAH even though the animals receive no treatment. An example of such a species is the marine fish scup (Stenotomus chrysops) which normally possesses high levels of certain hepatic monooxygenase activities, strong in inhibition of these by 7,8-benzoflavone, apparent selectivity in patterns of benzo[a]pyrene metabolism, and efficient activation of carcinogenic PAH to mutagenic derivatives. It is possible that these characteristics are associated either with a constitutive cytochrome(s) P-450 having unusual properties, or induction of scup chemicals in the environment. Evidence to date obtained with large numbers of scup, while not yet unequivocal, suggest that most of these features are a reflection of widespread induction of scup by environmental chemicals. The appearance in other species some characteristics of monooxygenase systems like those seen in scup might be an indication of pervasive effects of organic chemicals in the environment, even in the deep sea.