An alkalophilic bacterium belonging to the genus Bacillus was isolated from an indigo ball. The bacterium exhibited a maximum growth rate at pH 10-0 TO 10-5. The incorporation of 14C-labelled amino acids or [14C]uracil, uptake of 14C-labelled alpha-amino isobutyric acid into the bacterium and oxygen consumption of the bacterium with amino acids as substrates were all maximum at pH 9-0 to 10-5. The uptake of [U-14C]glucose into the organism and oxygen consumption with carbohydrates, on the other hand, showed little variation of rate in the pH 8 to 10 region. The oxygen consumption of intact bacteria or protoplasts in culture medium was maximum at pH 10. The membrane of the bacterium oxidized NADH maximally at pH 7-5, and ATPase bound to the membrane exhibited maximum activity at pH 7.L-Lactate, L-alanine and malate dehydrogenases in the soluble fraction exhibited maximum activities at pH 7-4 to 8-4. The alkalophilic property of the bacterium may be due to the behaviour of the membrane towards charged substances admitted into the organisms.