A cosmid clone complementing a cyclic beta-(1 --> 2)-glucan biosynthesis (ndvB) mutant of Rhizobium meliloti was isolated from a Bradyrhizobium japonicum gene library. This clone specified synthesis of beta-(1 --> 3),beta-(1 --> 6)-linked glucans in R. meliloti. The complemented strain was osmotically tolerant and symbiotically competent on alfalfa. Thus, beta-(1 --> 3),beta-(1 --> 6)-glucans can substitute functionally for beta-(1 --> 2)-glucans in R. meliloti.