Bacterial exopolysaccharide (EPS) is required for establishment of the nitrogen-fixing symbiosis between Rhizobium meliloti and its host plant, Medicago sativa (alfalfa), but the precise role of EPS in this interaction is not well defined. Bacterial mutants which fail to produce EPS induce nodules on the roots of the host plant, but fail to invade these root nodules. Research conducted in our lab and others suggests that EPS plays a specific role in the R. meliloti-M. sativa symbiosis. A common theme emerging from these studies is that small quantities of low-molecular-weight (LMW) EPS are sufficient to mediate successful invasion by R. meliloti mutants which fail to produce EPS, implying that LMW EPS may act as a signaling molecule during this process.