Among the several strains of halobacteria grown heterotrophically, ribulose bisphosphate carboxylase activity was detected in those which accumulate poly (β-hydroxybutyrate), viz. Haloferax mediterranei, Haloferax volcanii and Halobacterium marismortui. In H. mediterranei, the activity was present in cell extracts prepared after growth on a variety of carbohydrates. The ribulose bisphosphate carboxylase activity in H. mediterranei was inhibited by carboxyarabinitol bisphosphate, and the enzyme cross-reacted with antibodies raised against the spinach enzyme. CO2 fixation by cell extract was stimulated by the addition of ATP and NADH. Preliminary data suggested that hydrogen could be a possible reductant.