A molecular study of nodulation (nod) genes involved in soybean-Rhizobium fredii USDA193 symbiosis was conducted. The nod genes, nod ABC and nod D, respectively, were located on two unlinked DNA fragments. These two fragments could confer nodulation ability of soybeans to symbiotic plasmid deleted strains of R. fredii or to Ti plasmid-cured strain of Agrobacterium tumefaciens. One of these DNA fragments (a 2.8 kb EcoRI fragment) itself was capable of extending the host range of heterologous rhizobia.;Transposon mutagenesis studies have identified the putative host specificity gene coded by the 2.8 kb EcoRI fragment. Conservation of this host specificity gene sequences in several rhizobia was examined.;Preliminary studies identifying other regions involved in the nodulation process have been carried out. Such studies indicated the possibility of more than one host specificity gene present on unlinked DNA fragments.;DNA homology studies between Rhizobium and Agrobacterium strains, using an insertion sequence IS66 originally isolated from the Ti plasmid of A. tumefaciens, identified the presence of IS66 homologues in R. fredii genome. Such sequences were found to be clustered near the nif gene sequences of R. fredii DNA. Possible roles of these insertion-like sequences in the evolution of R. fredii plasmids and in the generation of spontaneous deletions or genetic rearrangements have been discussed.