The permanent meiobenthic community associated with aggregations of the tubeworm Riftia pachyptila was characterized at 2 different hydrothermal vent sites, Tica and Riftia Field, on the East Pacific Rise near 9 degrees 50'N. The maximum effluent temperatures were similar at both sites, but the chemistry of the hydrothermal fluids differed between sites. The abundance of meiobenthos was very low in 5 out of 6 samples (<61 ind. 10 cm(-2)) and was higher at Tica (20 to 976 ind. 10 cm(-2)) than at Riftia Field (< 1 to 12 ind. 10 cm(-2)). Meiobenthos abundance was positively correlated with the volume of sediment within the tubeworm aggregations. Sediment consisted mainly of particulate organic material and contained only a few mineral grains. A total of 33 meiobenthic species (15 of them new to science) was identified, comprising nematodes, copepods, ostracods, tanaidaceans, and foraminiferans. The meiobenthic fauna contributed a third to the total species richness in the benthic community associated with these tubeworm aggregations. There were 19 meiobenthic species shared between the 2 sites. The majority of meiobenthic species were first-order primary consumers. The most abundant taxa were nematodes and copepods, and other taxa were rare at both sites. Nematodes numerically dominated the community at Tica, while no clear dominance of a higher taxon could be detected at Riftia Field. Species richness was similar at both sites, whereas Shannon-Wiener diversity index and Pielou's evenness index were higher at Riftia Field. Due to the differences in the relative abundance of some species and unique occurrence of others at each site, the meiobenthic communities from the 2 different sites had an average Bray-Curtis dissimilarity of almost 70%.