ABSTRACT-Two species of deep-sea mussels belonging to the genus Bathymodiolus were collected from 5 vent fields distributed over 2 basins of a hydrothermal system in the South Pacific. Free and bound primary amino compounds were analysed in gills and mantles using OPA-HPLC. The method allowed the detection of 32 amino acids and related compounds (AARCs). An undetermined free chemical was found at hlgh levels (up to 30% of the total free AARCs) In vent mussel tlssues This amino compound is presumed to be an end-product from specific metabolism occurnng in vent n~ussels. Except for this unknown compound, comparlson of the biochemical composition between vent and littoral mussels revealed that the general nitrogenous requirement as well as the endogenous metabolic pathways of the main amino acids seem to be similar for the 2 mussel types. Ultrastructural observations indicated that these deep mussels harbour bacteria in their gills. Indices of high metabolic rates found in these organs appear to be related to the presence of endobacterial. It seems that symbionts could provide a preponderant fraction of essential amino acids to their host. In particular, free thiotaurine displayed high concentrations (up to 13% of the total free AARCs). This sulphur-containing compound could be involved in a specific metabolism related to toxic forms of sulphur. Variations of the AARC composition between the 2 vent mussel species do not appear to be species related but appear to be due to environmental constraints. As shown by stress indices, deterioration of the physiological condition in these mussels is directly hnked to the quantity and/or the quality of the hydrothermal vent emissions.