Supernodulating soybean (Glycine max L. Merr.) mutant SS2-2 and its wild-type counterpart, Sinpaldalkong 2, were examined for the microstructural events associated with nodule formation and development. SS2-2 produced a substantially higher percentage of curled root hairs than the wild type, especially at 14 days after inoculation with Bradyrhizobium japonicum. In addition, there was new evidence that in SS2-2, B. japonicum also entered through fissures created by the emerging adventitious root primordia. Early steps of nodule ontogeny were faster in SS2-2, and continued development of initiated nodules was more frequent and occurred at a higher frequency than in the wild type. These data suggest that the early expression of autoregulation is facilitated by decreasing the speed of cortical cell development, leading to the subsequent termination of less-developed nodules. The nodules of SS2-2 developed into spherical nodules like those formed on the wild type. In both the wild type and supernodulating mutant, vascular bundles bifurcate from root stele and branch off in the nodule cortex to surround the central infected zone. These findings indicate that SS2-2 has complete endosymbiosis and forms completely developed nodule vascular bundles like the wild type, but that the speed of nodule ontogeny differs between the wild type and SS2-2. Thus, SS2-2 has a novel symbiotic phenotype with regard to nodule organogenesis.