A complex interaction between rhizobia and specific legume plants results in the formation of nitrogen-fixing root nodules. The necessity for signal exchange and a chemically based recognition system between the symbiotic partners has been appreciated for some time, but the details are only gradually being elucidated. The two basic nodule ontogenies exhibit different requirements for Rhizobium exopolysaccharides. These surface exopolysaccharide molecules of Rhizobium are synthesized at a membrane complex, which is regulated by both transcriptional and post-trans-lational controls. The acidic exopolysaccharide probably plays both a passive and an active role during the invasion process.