In the genus Azospirillum tolerance towards high concentrations of sodium chloride, sucrose or polyethylene glycol increased in the order A. amazonense A. lipoferum A. brasilense and A. halopraeferens. In A. brasilense and A. halopraeferens the compatible solutes trehaloseglutamate and an unknown compound were identified. A. halopraeferens only could convert choline to the potent compatible solute glycine betaine. Acetobacter diazotrophicus tolerated high concentrations of sucrose and polyethylene glycol, but was very sensitive towards sodium chloride. In contrast to the more osmotolerant Azospirillum spp. amino acids such as glutamate, serine and histidine were efficiently utilized as carbon and nitrogen sources and betaine, choline and proline did not relieve osmotic stress. New halotolerant bacteria (strains BE and TC) were isolated from the rhizosphere of rice growing in alkaline, saline soil in India. They were oxidase-positive, Gram-negative, very motile bacteria, which showed pleomorphic growth. In semisolid nitrogen free mineral medium they grew and fixed nitrogen microaerobically. These isolates required sodium ions for growth and they tolerated up to 2 M sodium chloride in nitrogen containing mineral medium. At osmotic stress conditions the efficient compatible solute ectoine was synthesized.