Riftia pachyptila is one of the most specialized invertebrate hosts of chemoautotrophic symbionts. Crucial to the functioning of this symbiosis is how these worms cope with fluctuating ion concentrations. Internal sulfate levels in R. pachyptila appear comparable with other benthic marine invertebrates, despite the production of sulfate internally by means of the bacterial oxidation of hydrogen sulfide, suggesting that these worms are able to eliminate sulfate effectively. Internal chloride levels appear comparable; however, coelomic fluid chloride levels decrease significantly as the amount of coelomic fluid bicarbonate increases, demonstrating a 1:1 stoichiometry. We believe this shift in chloride, out of the body fluids, is needed to compensate for changes in electrochemical balance caused by the large increase (up to and greater than 60 mmol L-1) in negatively charged bicarbonate. Riftia pachyptila fits the general pattern of monovalent ion concentrations that is seen in other benthic marine invertebrates, with a high [Na+] : [K+] ratio extracellularly and low [Na+] : [K+] ratio intracellularly. Extracellular pH values of 7.38+/-0.03 and 7.37+/-0. 04 for coelomic fluid and vascular blood, respectively, as well as intracellular pH values of 7.37+/-0.04 and 7.04+/-0.05 for plume and trophosome tissue, respectively, were measured. On the basis of significant decreases in extracellular pH and, in some cases, Na+ and K+, in worms exposed to carbonyl cyanide m-chlorophenylhydrazone, sodium vanadate, and N-ethylmaleimide, we suggest that high concentrations of H+-ATPases, perhaps Na+/H+- or K+/H+-ATPases, are involved in H+ elimination in these animals.