Following transposon mutagenesis of Rhizobium leguminosarum biovar viciae, a mutant altered in its colony morphology was isolated. This mutant induced the formation of structures, superficially resembling root nodules on the bacterium's symbiotic host plant Vicia hirsuta. These structures were shown to be true nodules but were blocked in nodule development and as a consequence failed to reduce atmospheric nitrogen. An analysis of the surface polysaccharides produced by the mutant revealed that it failed to produce both the immunodominant 0-antigen moiety of the wild-type lipopolysaccharide molecule and a periplasmic B-glucan. The size distribution of the acidic exopolysaccharide differed markedly between the mutant and the wild-type strain even though these exopolysaccharides were structurally indistinguishable. The mutant's outer membrane was shown to be disorganized, presumably as a consequence of the absence of part of the lipopolysaccharide molecule which is thought to play a role in the maintenance of outer membrane integrity. Electron microscopic examination of the bacterial cells revealed that they possessed normal morphologies. The infection phenotype of the mutant strain of Rhizobium leguminosarum biovar viciae on Vicia hirsuta seedlings was normal indicating that neither 3-glucan nor lipopolysaccharide molecules are involved at the infection stage of nodule development Microscopic examination of sectioned material revealed that in the majority of cases the infection thread was limited to the nodule periphery. On a few occasions the infection thread did penetrate the central tissue of the nodule and in some of these cases bacteria were released. The penbacteroid membrane of these bacteroids was normal but the bacterial cell envelope had an altered topography which is consistent with the outer membrane of this mutant being disorganized. The plant cells into which the bacteria were released senesced rapidly. It is argued that lipopolysaccharide molecules play a role in the later stages of nodule development, most probably m bacteroid development. The Rhizobium leguminosarum biovar viciae mutant could be complemented by the introduction of a wild-type cosmid gene library. The gene responsible for the defect was shown to be chromosomally located, unlike many of the genes involved in the symbiosis which are plasmid encoded. The mutation was at a novel genetic locus and the very unusual phenotype exhibited by this mutant has not been described before.