Isolation and culture condition studies were conducted on bacteria capable of growing autotrophically on gaseous hydrogen. One of the isolates, the strain 9-5, showed the most rapid growth. Optimal growth occurred at pH 5.0 to 6.5. Ammonium or nitrate nitrogen was well utilized for growth, but urea and gaseous nitrogen could not be assimilated. Addition of 0.2mM ferrous ion was remarkably effective on bacterial growth. Three kinds of gases, H₂, CO₂ and CO₂ were almost completely assimilated when the ratio of the three components was 65:23:12, respectively. The strain 9-5 grew moderately even in an atmosphere containing 45% oxygen. A small amount of organic substrates such as glucose was shown to be effective at the initial stage of cell growth. As a result of investigations of culture conditions, cultured broth containing 12.4g/liter of dry cell was obtained within 48 hr.

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