A motility revertant of a Bradyrhizobium japonicum ndvB mutant was isolated and characterized. The ndvB mutants of B. japonicum have been reported to be osmotically sensitive, as well as defective in motility, periplasmic cyclic beta-(1-->3), (1-->6)-D-glucan synthesis, and symbiosis with soybean. The motility revertant was restored for osmotic tolerance but not for cyclic beta-glucan production or effective symbiosis. These results support our hypothesis that cyclic beta-glucans have an important role in symbiosis-the suppression of a plant defense response-in addition to their role in periplasmic osmoprotection.