The obligately anaerobic, moderately halophilic eubacteria Haloanaerobium praevalens (isolated from Great Salt Lake, Utah) and Halobacteroides halobius (isolated from the Dead Sea) were found to contain high intracellular potassium concentrations (0.76-2.05 M, not well correlated with the external NaCl concentration), and high intracellular sodium concentrations (0.28-2.6 M, increasing with increasing extracellular NaCl concentration). The sum of intracellular potassium and sodium concentrations approximated the total cation concentration of the medium. Internal chloride concentrations in Haloanaerobium praevalens equalled the external chloride concentration. Organic solutes, such as betaine, glycerol, and different amino acids that have been shown to serve as compatible solutes in other halophilic eubacteria and eukaryotic organisms, were not found in significant concentrations. Analysis of the amino acids in the proteins of the anerobic halophilic eubacteria showed an excess of acidic amino acids, similar to that reported in halophilic archaebacteria of the genus Halobacterium.