ABSTRACT: Hydrothermal-vent gastropods belonging to the genus Ifremeria of the family Provannidae, which derive their nutrition from chemoautotrophic bacterial endosymbionts, constitute an important faunal element in the ecology of deep-sea hydrothermal systems in the SW Pacific. In order to determine phylogenetic relationships between the hosts and endosymbionts of Ifremeria gastropods, as well as their fatty-acid profiles and bulk and compound-specific carbon-isotopic signatures, we analyzed Ifremeria gastropods from the Manus, North Fiji and Lau Back-Arc Basins in the SW Pacific. Partial sequences of the mitochondrial cytochrome c oxidase subunit I gene suggest that Ifremeria gastropods from the 3 basins belong to a single species, Ifremeria nautilei. Based on the 16S rRNA gene sequences and results from fluorescence in situ hybridization analysis, the gill endosymbionts of Ifremeria gastropods from the 3 basins were grouped in a single lineage of γ-Proteobacteria, with sequence similarities of >98.3%. Placement of the endosymbionts within this single lineage was supported by fatty-acid profiles and carbon-isotopic compositions of the Ifremeria gastropods. Phylogenetic relationships inferred among gastropod hosts and among their endosymbionts were not congruent, implying that acquisition of endosymbionts might be from the environment rather than through vertical transmission. Differences in geographical distribution and host speciation pattern between the confamilial Alviniconcha and Ifremeria gastropods might be attributed to the remarkable differences in symbiotic strategy with chemoautotrophic bacteria.