Water samples were collected around the communities of a hydrothermal mussel, Bathymodiolus sp., in the Lucky Strike (1700 m depth) and Menez Gwen (850 m depth) areas, Mid Atlantic Ridge, and analysed for chemical constituents. The environment surrounding the organisms consists in sea water (88 to 100%) mixed with hydrothermal fluid with pH between 6.2 and 8, Sigma S (H2S + HS-) concentrations from 0 to 62 mu mol l(-1). High dissolved organic carbon (DOC) concentrations suggest a highly productive ecosystem. Production of ammonium and DOG, consumption of nitrate and sulfide in the vicinity of the organisms are among the environmental changes found in relation with local biological activity. Concentrations of Cu and Pb were high, implying that the organisms need to have efficient strategies of adaptation. An empirical distinction between the extreme sizes of the mussels (> 6 cm and < 3 cm) reveals that the size distribution of the communities is related to the presence of different environments: the large size classes are present closer to the fluid exits, with higher Sigma S concentrations, than the small size classes. This could indicate differences in the growth rate related to the availability of the energy sources, competition for energy and space, and/or different settlement periods.