Strains of halophilic archaea, Haloarcula vallismortis and two Haloarcula strains OHF-1 and OHF-2, showed high tolerance to organic solvents at high media NaCl concentrations. For example, the lowest log P-ow of the solvent which allowed growth (log P-ow is the common logarithm of the partition coefficient of a given solvent in a mixture of n-octanol and water) for H. vallismortis was 5.1 at 20% NaCl and 4.4 at 30% NaCl. The solvent tolerance of Haloarcula argentinensis, on the other hand, was not affected by the NaCl concentration. Cells of strains OHF-1 and OHF-2 were of triangular or irregular morphology but became spherical in cultures in NaCl media overlaid with cyclohexane (log P-ow=3.4), but returned to the triangular shape when the organic solvent evaporated from the medium. When cells of strains OHF-1, OHF-2, and H. argentinensis were grown in NaCl media in the presence of n-decane, they contained less phosphatidylglycerol and more phosphatidylglycerosulfate and phosphatidylglycerophosphate methyl ester than when grown without added n-decane. When the solvent was removed from the media after cultivation, the levels of these compounds returned to their initial ones.