Rhizobium fredii USDA 206 harbors four large plasmids, one of which carries nodulation and nitrogen fixation genes. Previously isolated groups of plasmid-cured derivatives of strain USDA 206 were compared with each other to determine possible plasmid functions. Mutant strain 206CANS was isolated as a nonmucoid (Muc) derivative of strain 206CA, a mutant that was cured of two plasmids. The Muc phenotype of 206CANS was only expressed when the strain was grown on certain media, particularly those with polyols as carbon sources. Plasmid pRj206b of strain 206CANS was previously shown to have a higher copy number than the same plasmid in strains USDA 206 and 206CA. When this plasmid was transferred to Muc strains, it conferred a nonmucoid phenotype on recipient strains. The symbiotic effectiveness of the wild-type and cured strains was compared. Overall, few differences were shown, but strains 206CA and 206CANS were found to have higher nitrogenase activities than the other strains. Thus, there appeared to be a possible relationship among exopolysaccharide synthesis, plasmid copy number, and symbiotic effectiveness.