Benzo(a)pyrene (B(a)P) is a mutagenic polycyclic aromatic hydrocarbon (PAH) commonly released into the environment. B(a)P induces phase I biotransformation metabolism and peroxisome proliferation which is characterised in rodents by increased peroxisomal volume density, accompanied by the transcriptional induction of peroxisomal proteins. The aim of the present work was to study peroxisome proliferation at the transcriptional level, in comparison to the transcription of the well-characterised cytochrome P450 1A gene (cyp1a) in the thicklip grey mullet Chelon labrosus. For this purpose, genes coding for the major peroxisomal membrane protein PMP70 and CYP1A were cloned using degenerate primers. Then juvenile mullets were single injected with B(a)P (5mg/kg) and transcription of palmitoyl-CoA oxidase (aox1), multifunctional protein (mfp1), 3-ketoacyl-CoA thiolase (thio), Delta(2),Delta(4)dienoyl-CoA reductase 2, pmp70, catalase and cyp1a was semi-quantified in liver and gills 1 and 7days after the injection. Transcription of aox1 and cyp1a was induced in gills 1day after B(a)P injection. Cyp1a transcription was also induced in mullet liver one day after injection, indicating that B(a)P was available for the liver. This was further proved by the significant accumulation of B(a)P-like metabolites in bile 7days after the injection. In liver, aox1, mfp1 and thio transcription was induced at day 1 followed by the induction of catalase transcription at day 7 that may reflect a response to an oxidative stress caused by B(a)P itself or by oxyradicals produced through the biotransformation metabolism and the peroxisomal beta-oxidation. These hepatic responses were not reflected at AOX1 activity level. In conclusion, it has been shown for the first time that the three enzymes in the fish peroxisomal beta-oxidation pathway are transcriptionally induced by B(a)P. It remains to be tested whether this enhanced transcription is reflected in an increase in the volume of peroxisomes.