Anaerobic chemolithotrophic growth of Rhizobium japonicum occurred in the presence of carbon monoxide with nitrate serving as the electron acceptor. Under conditions where the atmospheric concentrations of CO were varied; the cells grew in up to 50% (v/v) CO. Optimum growth was observed in the presence of 20-30% CO. Under these conditions absorbance (660 nm) reached a maximum of 0.33 after 15 days of growth, after which no further increase was noted. Colony forming units increased from 1 X 10⁶ cells per ml to a maximum of 2.0 X 10⁸ cells per ml of culture medium. Growth in the absence of CO was substantially less indicating that CO was required for growth. The rate of disappearance of NO₃⁻-nitrogen from the culture medium during growth was correlated with the growth rate. Hydrogen uptake was measured amperometrically with cells grown in the presence of CO with nitrate, nitrite, nitrous oxide or oxygen serving as the electron acceptor. Addition of acetylene, in the presence of N₂O resulted in a 92.5% inhibition of N₂O-dependent H₂ uptake. Demonstration of H₂ uptake activity with NO₃⁻, NO₂⁻ or N₂O as the only electron acceptor substantiated the presence of dissimilatory nitrate, nitrite, and nitrous oxide reductase(s). Hydrogenase activity with O₂ as the electron acceptor in CO grown cultures showed the presence of cytochrome components necessary for transferring electrons from H₂ to O₂. Increasing the H₂ concentration above 0.5% (v/v) resulted in repression of growth in CO grown cultures. CO:oxidoreductase, the enzyme responsible for oxidation of CO was demonstrated in anaerobic, CO grown cultures. / M.S.