In R. meliloti we have identified four nodulation genes determining plant host-range specificity and have designated them hsnABC and D. The genes code for 9.7, 41.7, 26.7, and 28.6 kd proteins, respectively, and are organized into two transcriptional units. Mutations in these genes affect nodulation of their natural plant hosts Medicago sativa and Melilotus albus to different extents and hsnD mutants have an altered host-range. These Nod- mutations are not complementable by nodulation genes of other Rhizobium species such as R. leguminosarum. The hsn genes determine plant-specific infection through root hairs: hsnD is required for host-specific root hair curling and nodule initiation while the hsnABC genes control infection thread growth from the root hairs.