During the Biocyarise 1984 cruise, individuals of Alvinella pompejana and Bathymodiolus thermophilus were sampled by the submersible Cyana, on the 13 degree N site. Seventy-three heterotrophic bacterial strains were isolated from different body regions of these 2 animals. All the strains are Gram negative rods, most of them are motile. The bacteria isolated from Alvinella epidermis showed the highest index of carbohydrate and amino-acid utilization as sole carbon source. The thermal tolerance of these bacteria is well correlated to the temperatures of the biotopes of these invertebrates. The possible role of these heterotrophic bacteria in the hydrothermal ecosystem is discussed.