The phylogenetic diversity of the gut microbial population of the hydrothermal shrimp Rimicaris exoculata was determined. The presence of micro-organisms in the hindgut of the shrimp was determined, and their morphology illustrated for the first time by transmission electron microscopy. DNA was extracted from the fore-, mid- and hindgut of shrimps collected at the Rainbow site, at the Mid-Atlantic Ridge, and the sequences of the small-subunit rRNA (16S rDNA) were analyzed. Seven different bacterial phylotypes were identified from the 90 clones screened. The sequences were affiliated to three major groups: epsilon-Proteobacteria (48,8%), Entomoplasmatales (23.3%) and representatives of the new phylum Deferribacteres (26.6%). These results show that the diversity in the shrimp gut is very low compared to that of the surrounding medium. Furthermore, the presence of groups that are not found in the external medium (Spiroplasma sp. and Geovibrio sp.) suggests the existence of a local microflora. The potential roles of these bacteria are discussed, involving the shrimp diet and metal bioremediation.