Trophosome tissue of the hydrothermal vent tube worm, Riftia pachyptila (Vestimentifera), contains high activities of several enzymes associated with chemoautotrophic existence. Enzymes catalyzing synthesis of adenosine triphosphate using energy contained in sulfur compounds such as hydrogen sulfide, and two diagnostic enzymes of the Calvin-Benson cycle of carbon dioxide fixation, ribulosebisphosphate carboxylase and ribulose 5-phosphate kinase, are present at high levels in trophosome, but are absent in muscle. These data are consistent with an autotrophic mode of nutrition for this worm, which lives in hydrogen sulfide-rich waters and lacks a mouth and digestive system.