The potassium content ofHalobacterium halobiumwas studied in media in which the K+was growth-limiting and in media containing excess K+. In BSM medium (76.5 μg/ml K+) cells grew until all K+was cell bound. In BSMK medium (1.1 mg/ml K+) cells grew more abundantly; about one-third of the available K+became cell-bound. Adding glucose or vitamins to BSMK medium stimulated growth and could stimulate K+binding. However, rapid death in the presence of vitamins reduced the potassium-binding ability of most cultures. Living cells bound very large amounts of K+. After growth stopped in BSM medium they contained about 0.4 g K+per gram protein; that is, about 30% of the cell's dry weight consisted of K+. This is probably the lowest K+concentration supporting life of these cells. In BSMK medium, and in the BSM medium in the early stages of growth before all K+was cell-bound, considerably higher levels of K+were reached. Altering the pH or the NaCl concentration of the medium within the limits that still permitted growth did not affect the cells' K+contents.K+was released when cells in culture died. Non-growing cells incubated at 37 °C in salts solutions without K+or any energy source lived for up to 3 days, and with K+for at least 7 days. Their K+contents remained constant until they died, and then fell. If Mg2+was omitted from the salts solution cells died more rapidly than in the absence of K+. Dead cells did not lyse or release protein into the external medium.