Autoregulation of symbiotic root nodulation in soybean seedlings (Glycine max L. Merrill cv Pride 216) was studied following double inoculation of primary roots with Bradyrhizobium japonicum 110. When the second inoculation was given 10 or 17 hours after the first, the nodulation in the first-inoculated region of the root was suppressed. The effect was eliminated if B. japonicum 110 containing Tn5 insertions in the ;common' nod ABC genes was used for the second inoculation, indicating the requirement for changes in the root mediated by these bacterial genes. When the root cortex in the suppressed basal region was examined 3 days after inoculation, cell division centers were present in numbers not significantly different from the numbers in control roots given a sham second inoculation; their size distribution, however, showed a failure of enlargement compared with controls.