A full length cDNA coding for cytochrome P450 1A1 was isolated from a plaice liver cDNA library constructed in lambda ZAPII. The deduced amino acid sequence of this cDNA was 78% homologous to that of rainbow trout P450 1A1 and 57 and 51% homologous to human P450 1A1 and P450 1A2, respectively. Comparisons of these sequences show the plaice cDNA to be most similar to mammalian and trout P450 1A1 sequences, but also to have certain residues specific to fish P450 1A1. Analysis of Southern blots of restriction endonuclease-digested plaice genomic DNA showed only 1 or 2 bands after hybridization to the first 800 nucleotides of the plaice cDNA, and Northern blots showed only one cross-hybridizing band in a variety of tissues, suggesting that only 1 gene and no closely related sequences were present. This evidence implies that the plaice (and trout) P450 may represent an ancestral gene to the mammalian P450 1A1 and P450 1A2 genes. A variety of plaice tissues showed increases in P450 1A1 RNA after treatment with the prototypical P450 1A family inducers (3-methylcholanthrene [3MC] and isosafrole [ISF]); the strongest response occurred in heart, with slightly less induction in kidney and intestinal mucosa. A significant induction of P450 1A1 mRNA was also observed in plaice whole blood after 3MC or ISF treatment. Liver P450 1A1 mRNA showed a relatively weak response to these inducers but did exhibit elevated microsomal EROD activities, especially in ISF-treated animals. The results are discussed with reference to the possible use of plaice P450 1A1 cDNA as a probe for environmental monitoring studies.