Abundant bacteria have been found on the abfrontal regions of the gill filaments of three species of small mussels from a variety of habitats: Idas washingtoniana and Adipicola sp. from hydrothermal vents on the Juan de Fuca Ridge; Idas simpsoni from a whale skull and from oily drill cuttings in the North Sea. The ultrastructure of their gills and of the associated bacteria is described and illustrated. The Adipicola species differs from the two species of Idas in having bulkier gills and a thicker layer of bacteria, with tighter retention of the symbionts under stalked clusters of microvilli. In both genera the symbionts are superficial, similar to the situation in some Thyasiridae, but unlike the enclosed symbioses of Bathymodiolus species, Lucinidae or Vesicomyidae. The bacterial ultrastructure indicates thiotrophic rather than methylotrophic metabolism in each case. The habitats and food sources of these small mussels are discussed and it is suggested that absorption of dissolved organic compounds may play a part in their nutrition in some circumstances.