Growth of Halobacterium cutirubrum, H. salinarium, and H. halobium was followed by viable counts and optical density measurements. The last species, which grows most vigorously, was most intensively studied.The increase of optical densities and the increase in viable counts corresponded through the exponential growth phase. Dead cells did not lyse, and viable counts were needed to show that after the stationary phase was reached some cultures died off rapidly. Others remained stable for several days. Potassium can limit growth in synthetic media; in all media abundant growth could be obtained only when about 1 mg/ml K+ was present. Growth was stimulated by glycerol, certain carbohydrates, related metabolites, and vitamins. The most effective growth-promoting vitamins tested were thiamine, folic acid, biotin, and vitamin B12. Metabolites that stimulated growth were galactose, glucose, sodium lactate, and glutamine. One-tenth percent glycerol, sodium succinate, and sodium pyruvate were stimulatory, although 2.0% concentrations were inhibitory. Though carbohydrates may stimulate growth, the medium does not become acid; this may explain why standard bacteriological tests indicate that many extreme halophiles do not "utilize" carbohydrates. H. halobium produced considerable acid only when grown in the presence of 2% glycerol.