Random Tn5 insertional mutants were induced inRhizobium meliloti Rmd201, a streptomycin-resistant mutant of AK631 (which is itself a compact colony morphology mutant of the wild-type strain Rm41), and screened for sensitivity to a set of 16 phages. Out of 3000 mutants 240 were found to be phage-resistant. The phage-resistant mutants were separable into six groups on the basis of their sensitivity pattern against test phages. Nodulation tests on alfalfa showed that although all the phage-resistant mutants induced root nodules, 7 mutants out of 12 of a class resistant to phage RMP64 (Sxf-) induced atypical nodules that were ineffective in nitrogen fixation (Fix-). The aberrant nodules were small, white, contained only a few bacteria and no bacteroids, and phenotypically resembled nodules elicited by already knownexoB, exoH, ndvA andndvB mutants ofR. meliloti. Spontaneous mutants selected for resistance to RMP64 also fell into two groups: Fix+ and Fix-. Genetic complementation tests between the Sxf- mutants defined three genessxfA, sxfB andsxfC, of whichsxfA andsxfB comprise an operon. These also demonstrated thatsxfA, sxfB andsxfC must be located on the same replicon. All the Sxf- mutants were Calcofluor-positive, like their parent strains Rmd201 and AK631. Characterization of carbohydrate metabolism of the mutants revealed that while thesxfA (Fix-) andsxfB (Fix+) mutants utilized galactose as sole carbon source,sxfC (Fix-) mutants did not. It has been concluded thatsxf A, sxfB andsxfC are new genetic loci and thatsxfA andsxfC have roles in nodule invasion and development.