Several purine auxotrophs were isolated inRhizobium meliloti and characterized for their nutritional requirements. They were found to produce small, irregular nodules lacking any detectable nitrogenase activity onMedicago sativa. The symbiotic aberration manifests itself only in the late developmental stage, for, (i) these purine auxotrophs infect theMedicago sativa root hairs by forming normal infection threads, and (ii) the mutants are recovered from the root nodules induced by them. External supplementation of the plant growth substrate with purines or their biosynthetic intermediates fails to restore symbiosis. This, and the failure of complementation of these auxotrophs with the known symbiotic genes, demonstrates that these mutants perhaps define a new set of genes influencing the symbiotic process inRhizobium meliloti.