We have previously reported a soybean plant introduction, PI 437153A, which was ineffectively nodulated (definition of G. Vest, D. F. Weber, and C. Sloger, Agronomy 16:353-390, 1973) by Bradyrhizobium japonicum strain UMR (University of Minnesota Rhizobium) 161. In this study, we further characterize this nodulation system by (i) examining the nodulation of 21 B. japonicum strains, representing 15 serogroups, with PI 437153A, (ii) ascertaining the inheritance of the ineffective nodulation, and (iii) characterizing the symbiosis both morphologically and physiologically. Only USDA 126 (serogroup 125) and UMR 161 failed to produce effective nodules with PI 437153A. Segregation among F2 and F3 plants from a cross between cv. Lambert and PI 437153A was consistent with control of this trait by a single, dominant gone. In reciprocal grafts between cv. Lambert and PI 437153A, ineffective nodulation was determined by the root genotype, and was independent of plant growth temperature and inoculant concentration. The nodule dry weight of UMR 161-inoculated plants of PI 437153A plants was approximately 10-fold less than that of comparable plants inoculated with USDA 110; nodule soluble protein levels were also significantly reduced. Nitrogenase activity and leghemoglobin could not be detected in the ineffectively nodulated phenotype, but 1 to 6 days after inoculation (DAI) phenylalanine ammonialyase RNA level was higher in the ineffectively nodulated phenotype than in the effectively nodulated one. Microscopic examination of effectively, and ineffective nodules 20 and 35 DAI revealed major morphological differences. Ineffective nodules contained large, deteriorating regions near apparently infected plant cells at 35 DAI, and schlerenchyma cells that were stained red, suggestive of lignin deposition. These changes could be the result of incompatibility between host and Bradyrhizobium, though the lack of chalcone synthase expression and the appearance of infected cells within the ineffective nodules suggest that some later steps in the infection process do occur.