Seven strains of extremely halophilic bacteria (Halobacterium spp., Halococcus spp., and Haloarcula sp.) fixed CO2 under light and dark conditions. Light enhanced CO2 fixation in Halobacterium halobium but inhibited it in Halobacterium volcanii and Haloarcula strain GN-1. Propionate stimulated 14CO2 incorporation in some strains, but inhibited it in others. Semi-starvation in basal salts plus glycerol induced enhanced CO2 fixation rates. 14CO2 fixation in semi-starved cells was stimulated by NH 4 + or pyruvate and inhibited by succinate and acetate in most strains. No possible reductant was found. In cell-free extracts of H. halobium, NH 4 + but not propionate stimulated 14CO2 fixation. No RuBP carboxylase activity was detected. The main 14C-labeled α-keto acid detected after a 2-min incubation with 14CO2 and pyruvate was pyruvate. Little or no α-ketobutyrate was detected among the early products of propionate-stimulated CO2 fixation. Glycine was the major amino acid synthesized during a 2-min incubation with NH 4 + , propionate, and 14CO2. Propionate-stimulated CO2 fixation was sensitive to trimethoprim and insensitive to avidin. A novel pathway for non-reductive CO2 fixation involving a glycine synthase reaction with CO2, NH 4 + , and a methyl carbon derived from the β-carbon cleavage of propionate is tentatively proposed.