Cyclic-beta-glucans (C beta G) consist of cyclic homo-polymers of glucose that are present in the periplasmic space of many Gram-negative bacteria. A number of studies have demonstrated their importance for bacterial infection of plant and animal cells. In this study, a mutant of Rhizobium ( Sinorhizobium) sp. strain NGR234 (NGR234) was generated in the cyclic glucan synthase ( ndvB )-encoding gene. The great majority of C beta G produced by wild-type NGR234 are negatively charged and substituted. The ndvB mutation abolished C beta G biosynthesis. We found that, in NGR234, a functional ndvB gene is essential for hypo-osmotic adaptation and swimming, attachment to the roots, and efficient infection of Vigna unguiculata and Leucaena leucocephala.