The extremely halophilic bacterium (formerly designated as strain J.F. 54) isolated from salt pools of the Death Valley, California, is a motile, Gram-negative, extremely pleomorphic organism, aerobe, and facultative anaerobe. A variety of carbohydrates are assimilated with or without acid production; soluble starch is hydrolyzed. The organism is not proteolytic; catalase, oxidase, and DNase reactions are positive; Tween 20 is slightly hydrolyzed, but Tweens 40, 60, and 80 are not. Nitrates are reduced to nitrites with gas production; nitrites are not reduced. Optimum growth temperature is 40 degrees C. Growth is inhibited by bacitracin and by novobiocin. The type strain J.F. 54 differs from described species of the genus Halobacterium and is assigned to a new species, Halobacterium vallismortis sp. nov.