We carried out a 16S rDNA-based molecular survey of the prokaryotic diversity associated with the chitin tubes of the giant vent tubeworm Riftia pachyptila (collected at the East Pacific Rise, 9°N and 13°N). Scanning electron microscopy showed dense microbial populations, particularly on the external surface of the middle and upper tube regions, which included very diverse prokaryotic morphotypes. We used archaeal- and bacterial-specific primers for polymerase chain reaction (PCR) amplification, but only bacterial amplicons were obtained. We analysed a total of 87 clones. Most belonged to the ɛ-Proteobacteria, but also to the δ-, α- and γ-Proteobacteria. A broad diversity of phylotypes belonging to other bacterial divisions was detected, including Verrucomicrobia, the Cytophaga–Flavobacterium–Bacteroides group and the candidate division OP8. We also retrieved a sequence, R76-B150, of uncertain phylogenetic affiliation, which could represent a novel candidate division. The sequence of the R. pachyptilaγ-proteobacterial endosymbiont was not detected. The bacterial diversity found suggests that complex metabolic interactions, particularly based on sulphur chemistry, may be occurring in different microniches of the R. pachyptila tubes.