The biotransformation of 1-methylnaphthalene and anthracene has previously been investigated in bacteria but no data exist on mussel. The experiments were made on previously depurated mussel in artificial seawater at room temperature, protected from light, not aerated and not fed. After addition of 1-methylnaphthalene and anthracene separately in the tanks, the analyses were made on mussel and in water by HPLC and GC/MS. The biotransformation products determined in mussels were: 1-naphthoic acid on the 6(th) day and 9, 10-anthraquinone on the 2(nd) day. The release of xenobiotics was detected in water, 1-naphthoic acid on the 7(th) day and 9, 10-anthraquinone on the 4(th) day. This is the first record on biotransformation of tested compounds in mussels.