To facilitate genetic analyses of Rhizobium meliloti genes that are involved in symbiosis, we determined the map positions of 11 symbiotic loci on the R. meliloti chromosome by using a combination of the Tn5-Mob conjugational transfer method described by Klein et al. (S. Klein, K. Lohmann, G. C. Walker, and E. R. Signer. J. Bacteriol. 174:324-326, 1992) and co-transduction of genetic markers by bacteriophage phi M12. Loci involved in effective nodule formation (fix-379, fix-382, fix-383, fix-385, and fix-388), polysaccharide synthesis (exoR, exoS, exoC, and ndvB), nodule invasion (exoD), and nitrogen regulation (ntrA) were ordered with respect to previously mapped markers and each other. The positions of two other loci, degP and pho-1, were also determined.