Polycyclic aromatic hydrocarbons (PAHs) are a group of environmental contaminants widely studied for their high toxicity in distinct life forms, including humans.Benzo[a]pyrene [B(a)P], one PAHs, is ranked number 8 (of 275 chemicals) on the Priority List of Hazardous substances for humans.In order to understand biotransformation mechanisms of B(a)P, the biochemical response of the sea anemone Anthopleura elegantissima to B(a)P was evaluated in the present work.A bioassay was performed using three groups injected with 100 µl of 2-, 4-, and 6-mM B(a)P, and two controls treated with either 100 µl of seawater or solvent (named GC and GS).After 72 h, nuclear, mitochondrial, and microsomal fractions were prepared from the columnar tissue, and NADPH cytochrome C reductase was measured.Enzyme activity in GC organisms was 1.135 ± 0.006, 2.95 ± 0.04, and 8.913±0.13nmol min-1 mg-1 in nuclear, microsomal, and mitochondrial fractions, respectively, with no difference between control groups.For the B(a)P treated organisms, only the microsomal fraction showed a correlation between the toxic and NADPH cytochrome C reductase.The strong positive correlation between pollutant and the biochemical response point to the inducibility of the enzyme by benzo(a)pyrene in A. elegantissima.