The sulphur compound composition of the hydrothermal vent mussel Bathymodiolus azoricus (Bivalvia: Mytilidae) was analysed in order to determine the speci¢c biochemical characteristics of a thiotrophic mode of nutrition. In specimens collected from two contrasting vent-¢elds on the Mid-Atlantic Ridge (Menez Gwen and Lucky Strike), substantial di¡erences in the sulphur composition were observed between tissues and sampling sites. Total sulphur content was higher for samples from Lucky Strike than for those from Menez Gwen. The amount of elemental sulphur in the gill of B. azoricus was in the same range as values previously reported for the vesicomyid clam Calyptogena phaseoliformis and the lucinid clam Lucinoma annulata. Rings of orthorhombic sulphur or compounds such as polythionates or organopolysul-phides excreted in the cytoplasm by the symbionts may account for the large amounts of elemental sulphur evidenced for the ¢rst time in an hydrothermal vent mussel. A large proportion of the tissue-sulphur was incorporated into free amino compounds such as taurine, hypotaurine, thiotaurine and cysteine, and in a lesser extent to the tripeptide glutathione. In mantles, sulphur seems to be contained mainly in organic compounds such as proteins, mucopolysaccharides and lipids. However, the occurrence of speci¢c compounds such as thiotaurine and elemental sulphur in gills indicate that in this tissue a large proportion of the sulphur might be involved in speci¢c pathways related to transport, storage and detoxi¢cation of sulphide. Moreover, our results suggest a greater reliance on thiotrophy of mussels from Lucky Strike as compared to specimens from Menez Gwen.