Exogenous H-2/CO2 and glucose were consumed simultaneously by Butyribacterium methylotrophicum when grown under glucose-limited conditions. CO2 reduction to acetate was coupled to H-2 consumption. The addition of either H-2 or CO2 to glucose batch fermentation resulted in an increase in cell density hydrogenase (H-2-consuming and -producing) activities and fatty acid production by B. methylotrophicum as compared to when N-2 was the feed gas. Hydrogenase activities appeared to be tightly regulated and were produced at higher rates during the exponential phase when CO2 was the feed gas as compared to H-2 or N-2. The increase in H-2-consuming activity and decrease in H-2-producing activity was correlated with an increase in butyrate synthesis. H-2-consuming and ferredoxin (Fd) NAD reductase activities increased while H-2-producing and NADH-Fd reductase activities decreased in cells grown at pH 5.5 compared to those at pH 7.0. The molar ratio of butyrate/acetate was shifted from 0.35 al pH 7.0 to 1.22 at pH 5.5. The addition of exogenous H-2 did not decrease the butyrate,acetate ratio at pH 7.0 not at pH 5.5, The results indicated that growth pH values regulated both hydrogenase and Fd-NAD oxidoreductase activities such that, at acid pH, more intermediary electron flow was directed towards butyrate synthesis than H-2 production.