1. 133 strains of actinomycetes comprising 11 genera were tested for their ability to grow chemolithotrophically; 9 strains were stimulated by hydrogen on an organic medium under reduced partial pressure of O2 (8%). 2. Even after repeated subculturing, a number of strains grew well on mineral medium with H2 as the sole energy source and CO2 as the only carbon source: Mycobact. phlei (1 strain), Noc. saturnea (2 strains), Noc. petroleophila (2 strains), Noc. [Strept.] autotrophica (1 strain), Streptomyces spec. (1 strain) and Streptosporangium spec. (1 strain). 3. Three strains more intensively investigated (Nocardia 99, 102 and 394) should be regarded as facultative chemoautotrophs in view of their (1) ability to activate molecular hydrogen (methylene blue [“MBL”] reduction), (2) catalyzing a Knallgas-reaction (oxyhydrogen reaction), (3) utilization of the energy gained from the Knallgas reaction to incorporate CO2 in a highly reduced state (Knallgas reaction linked to CO2 fixation), and (4) ability to grow on mineral medium with H2−O2−CO2 as the sole source of carbon and energy. 4. The hydrogenase activity of the three strains was approximately equal; the Knallgas reaction took place only in living cells. 5. Although the velocity of gas uptake was greater with MBL than with O2 (−CO2!), in the presence of O2 MBL retarded the gas uptake and inhibited CO2 fixation. Resting cells in the presence of CO2 showed increased velocity of Knallgas uptake, as compared with cells kept without CO2. In these experiments the average economical coefficient (μM H2 oxidized/μM CO2 fixed) was about 13. 6. Cells grown heterotrophically exhibited only a low hydrogenase activity; induced formation of hydrogenase was slow. Cells grown chemolithotrophically could be harvested after 18 days to give a yield of 750 mgs (cell dry weight) per 3 liter mineral medium with one of the culture methods described.