We have identified a set of Tn5-generated mutants of Rhizobium meliloti on the basis of their failure to form a fluorescent halo under UV light when grown on agar medium containing Calcofluor. These mutations define a new genetic locus we have termed exoH. Alfalfa seedlings inoculated with exoH mutants form ineffective nodules that do not contain intracellular bacteria or bacteroids. Root hair curling is significantly delayed and infection threads abort in the nodule cortex. Analyses of exopolysaccharide secreted by exoH mutants have shown that it is identical to the Calcofluor-binding exopolysaccharide secreted by the exoH+ parental strain except for the fact that it completely lacks the succinyl modification. In vitro translation of total RNA isolated from nodules induced by an exoH mutant has shown that only one of the plant-encoded nodulins is induced, as compared with the 17 nodulins induced by wild-type strains. These observations suggest that succinylation of the bacterial polysaccharide is important for its role(s) in nodule invasion and possibly nodule development.