The xenobiotic detoxification enzymes are regulated, in mammals and other animals, by a specific group of nuclear receptors. In particular pregnane X receptor (PXR) has shown incredible versatility in recognizing a broad range of xenobiotics and natural compounds which function as agonists of the protein. However this regulatory mechanism is poorly characterized in marine invertebrates including bivalves. The objective of this work was, 1) to collect sequence information from PXR homologous genes already discovered, 2) to search for putative PXR homologous sequences in other marine species and in the marine mussel Mytilus galloprovincialis.