Aims: To characterize the physiological and metabolic responses of Mesorhizobium ciceri strain ch-191 to salt stress, investigating the changes induced by salinity in protein and lipopolysaccharide profiles, as well as determining the accumulation of amino acids, glutamate and proline. Methods and Results: Strain ch-191 of M. ciceri was grown with different NaCl concentrations. Protein and lipopolysaccharide patterns were determined by electrophoresis. The strain ch-191 tolerated up to 200 mmol l(-1) NaCl, although higher salt dosages limited its growth and induced changes in the protein profile. The most noteworthy change in the LPS-I pattern was the decrease in the slowest band and the appearance of an intermediate mobility band. The accumulation of proline in response to salt stress surpassed that of glutamate. Conclusions: The protein profile showed major alterations at salinity levels which inhibited growth. However, the alterations in the LPS profile and accumulation of compatible solutes were evident from the lowest levels, suggesting that these changes may constitute adaptative responses to salt, allowing normal growth. Significance and Impact of the Study: The selection and characterization of salt-tolerant strains, which also show efficient symbiotic performance under salinity, may constitute a strategy for improving Cicer arietinum-Mesorhizobium ciceri symbiosis in adverse environments.