Gastropods of the genus Alviniconcha are found at hydrothermal vent fields in the Western Pacific, and have been reported to associate with either γ- or ε-Proteobacterial endosymbionts.  These symbionts harness energy from the oxidation of chemicals in vent fluid to fix inorganic carbon and are the primary source of nutrition for the host.  An extensive sampling effort from a recent expedition to the Eastern Lau Spreading Center (ELSC) has revealed that Alviniconcha with symbionts of both types are found at vent fields along the ridge and that the dominance of each type at a vent field relates to the geology and geochemistry of that site.  We collected 266 Alviniconcha individuals from four vent fields along the spreading center (30-140km apart) which span the north-south transition from fast spreading, basalt-hosted vent fields to slower spreading, andesite-hosted fields.  Vent fluids from each field were also analyzed for the abundances of aqueous volatile and non-volatile species.  The symbionts of all collected Alviniconcha were genotyped using restriction fragment length polymorphism analysis of the 16S rRNA gene.  Individuals were found to host one of three symbiont genotypes (two γ- and one ε-Proteobacteria) and 16S rRNA genes from representative individuals of each type were sequenced.  We found that the two northern-most sites (basalt) were dominated by individuals with the ε-Proteobacterial symbiont, while the two southern sites (andesite) were dominated by individuals hosting one of the two γ-Proteobacterial symbionts.  This pattern also corresponds to differences in the aqueous chemistry of the vent fluids along the spreading center: in particular, we have measured higher concentrations of hydrogen and hydrogen sulfide in the vent fluids at the northern sites than in the fluids of the southern sites.  Thus, vent chemistry may be influencing the dominance of each symbiont type along the ELSC.