Deep-sea hydrothermal vents are home to a variety of invertebrate species, many of which host chemosynthetic bacteria in unusual symbiotic arrangements. The vent tubeworm Riftia pachyptila (Vestimentifera) relies upon internal chemolithoautotrophic bacterial symbionts to support its large size and high growth rates. Because of this, R. pachyptila must supply sulfide to the bacteria, which are far removed from the external medium. Internal ΣH₂S ([H₂S+HS⁻+S²⁻]) can reach very high levels in R. pachyptila (2–12 mmol l⁻¹ in the vascular blood), most of which is bound to extracellular hemoglobins. The animal can potentially take up sulfide from the environment via H₂S diffusion or via mediated uptake of HS⁻, or both. It was expected that H₂S diffusion would be the primary sulfide acquisition mechanism, paralleling the previously demonstrated preferential uptake of CO₂. Our data show, however, that the uptake of HS⁻ is the primary mechanism used by R. pachyptila to obtain sulfide and that H₂S diffusion into the worm apparently proceeds at a much slower rate than expected. This unusual mechanism may have evolved because HS⁻ is less toxic than H₂S and because HS⁻ uptake decouples sulfide and inorganic carbon acquisition. The latter occurs via the diffusion of CO₂ at very high rates due to the maintenance of an alkaline extracellular fluid pH. ΣH₂S accumulation is limited, however, to sulfide that can be bound by the hemoglobins, protecting the animal from sulfide toxicity and the symbionts from sulfide inhibition of carbon fixation.