The cyanobacterium Nostoc muscorum is able to utilized proline and glycine-betaine as a nitrogen source under unstressed growth conditions. This cyanobacterium when grow in modified Chu No. 10 medium (without Na+) unable to utilized proline and glycine-betaine as a nitrogen source. Spontaneously occurring mutant clones defective in Na+ transport (Na+-R) were isolated and analyzed for proline and glycine-betaine utilization. The mutant phenotype showed normal heterocyst frequency and nitrogenase activity even in the medium containing 1 mM proline or 1 mM glycine-betaine, indicates the role of Na+ for proline/glycine-betaine uptake. The Na+-R mutant showed 100% survival at pH 11 and was simultaneously able to uptake and utilize proline/glycine-betaine at higher alkaline pH. This indicates that proline and glycine-betaine uptake systems are more efficient at higher alkaline pH. Since, the hypersaline environments are rich in Na+ contents and have alkaline pH, therefore it is suggested that the origin and evolution of specific compatible solutes may not depend only on the osmoregulatory role they play, but also on the other ecological factors operating simultaneously in the organism's niche.