Cytochrome P450 family (CYP) is a group of proteins virtually found in all living organisms. The main role of most CYPs is to metabolize endo and xenobiotics. Most of the studies on CYP have been carried out in mammals and other vertebrates, however recently a growing interest has been devoted to the identification of CYP isoforms in invertebrates. A gene belonging to the CYP sub-family, CYP356A1, was identified in sanitary sewage-exposed Pacific oysters, Crassostrea gigas. Through heterologous expression, we produced CYP356A1 purified protein and raised a mouse polyclonal antibody. Dot blot tests showed that oysters exposed in situ for 14 days to untreated urban effluent discharges had significantly higher levels of CYP356A1 in digestive gland. Using immunohistochemical techniques we observed that the lining epithelial cells of mantle, stomach and intestine showed a strong CYP356A1 staining, but the mucus and secretory cells were negative. Digestive diverticulum parenchyma and gills lining cells showed strong CYP356A1 reaction, while the filamentary rod (connective tissue) was negative. Free cells, as hemocytes and brown cells also showed CYP356A1 immunoreactions indicating the presence of biotransformation activity in these cells. Male germ cells at early stages expressed CYP356A1 but not sperm mature cells, suggesting that this protein could be involved in the male gonadal development. This study shows the use of a specific antibody to a mollusk CYP isoform and that this protein is inducible in oysters environmentally exposed to urban sewage effluents.