A procedure is described for the isolation of mutants of Azospirillum brasilense strains Sp7 (ATCC 29145) and Sp245 and Azospirillum lipoferum Br1702, which are unable to differentiate from vegetative vibrioids into encapsulated C forms. It is based on the characteristic of mutants, designated Enc−, to develop into white colonies in a background of wild-type red colonies (Enc+) on the surface of Congo Red plates. This behavior was ascribed to an inability of Enc− mutants to synthesize one or more as yet unidentified polysaccharides. The frequency of mutation from Enc+ to Enc− was a function of the particular strain and of the culture conditions employed. Enc− mutants from A. brasilense and A. lipoferum were at least as efficient as wild-type ancestors in adhering to and causing deformations of root hairs of germinated wheat seedlings.