Phosphoglucomutase is a key enzyme in glycolysis and has been widely studied in vertebrates and some invertebrates but no molecular information is available in marine invertebrates despite the importance of this marker in ecological and genetical studies. In this work, we isolated a cDNA and the corresponding genomic sequence that encode PGM-2 locus in the Pacific oyster Crassostrea gigas. We used sequences drawn from the database to construct an evolutionary framework for examining the position of mollusc PGM sequences among prokaryotic and eukaryotic homologues and showed that oyster PGM gene organization was closer to vertebrates PGM genes than other invertebrates as previously found in other Lophotrochozoa species. We also investigated PGM mRNA expression in oyster tissues in response to xenobiotics (i.e hydrocarbons and pesticides). The results obtained showed that PGM mRNA expression is mostly up-regulated in the first steps of the response to pollutant exposure and is xenobiotic-dependant.