Summary: Walls of the bacteroid form of Rhizobium leguminosarum, from plant nodules, and walls of free-living R. leguminosarum were isolated and compared by extraction with aqueous phenol and chemical analysis. Bacteroid walls had less lipopolysaccharide than walls isolated from bacteria cultured on four different media. This difference may be relevant to the relatively greater permeability of the outer membrane of bacteroids, and important in symbiotic interactions.