A heat-stable endogenous CO(2) acceptor has been found in extracts of Rhodospirillum rubrum grown photoheterotrophically on acetate. Evidence is presented which suggests that this factor is propionic acid. Thus, paper and gas chromatographic analyses have indicated that propionic acid is present in boiled extracts prepared from R. rubrum cells. The products of (14)CO(2) fixation obtained with either the boiled extract or propionic acid as the CO(2) acceptor were identical and were identified as methylmalonic acid and succinic acid by paper chromatography. The enzyme which catalyzes the carboxylation of propionyl-coenzyme A (propionyl-CoA carboxylase) was purified from R. rubrum cells grown on acetate and its properties were studied. The enzyme is similar to propionyl-CoA carboxylases isolated from mammalian sources.