A survey of the results of different ultrastructural and biochemical studies on bivalve molluscs found in dense populations on the sites of deep hydrothermal vents of the East Pacific Ridge shows that these organisms demonstrate specific ecological and biological features. The most remarkable characteristic of all species is a well-developed gill, constituting the main organ in the nutritive processes, with a majority of its cells colonized by abundant chemoautotrophic bacteria. Structural modifications related to the utilization of particulate material, as well as structural adaptations attributed to bacterial colonization of the gill, are different in the Vesicomyidae Calyptogena magnifica and the Mytilidae Bathymodiolus thermophilus and Bathymodiolus sp. These differences in the degrees of adaptation indicate several possible nutritive pathways.