Halobacteria live in extremely hypersaline environments accumulating K+ as compatible solute. We have studied the accumulation of intracellular potassium in recently isolated halobacteria with relatively moderate salt response, and compared it with the classical halobacteria with very extreme salt response. Significant differences have been found, the more moderate group having lower intracellular K+ concentrations. Some experiments have been carried out concerning the energy dependence of this K+ accumulation, which indicate that the moderate group seems to be less dependent on metabolic energy. The results are consistent with existence of different degrees of halophilia among halobacteria, probably corresponding to adaptation to different habitats.