Since N03-availability in the rooting medium seriously limits symbiotic N2 fixation by soybean (Glycine max [L.] Merr.), studies were initiated to select nodulation mutants which were more tolerant to N03-and were adapted to the Midwest area of the United States. Three independent mutants were selected in the M2 generation from ethyl methanesulfonate or N-nitroso-N-methylurea mutagenized Williams seed. All three mutants (designated NODI-3, NOD2-4, and NOD3-7) were more extensively nodulated (427 to 770 nodules plant-1) than the Williams parent (187 nodules plant1) under zero-N growth conditions. This Since the nts mutants selected by Gresshofls group were derived from the cultivar Bragg which is a Maturity Group VII line and cannot be grown to maturity under Midwest growing conditions, the current study was initiated to select nodulation mutants from a background which can be field tested in the major Midwest soybean production area. This paper provides the initial characterization of three soybean lines with enhanced nodulation capability which were selected from mutagenized populations of Williams (Maturity Group III). MATERIALS AND METHODS Plant MaterialSoybean (Glycine max [L.] Merr., cv Williams) seeds were mutagenized with EMS or NMU as previously described (17). In brief, the four mutagen-treatments (two chemicals and two postwash intervals) consisted of presoaking all seed for 16 h in vigorously aerated water, then treating separate seed lots for 2 h with 50 mM EMS or 5 h with 2.5 mm NMU, followed by either 5-or 9-h postwashes. The mutagenized seed (designated M,) was planted in the field and, at maturity, the four M2 seed lots were individually bulk harvested. The M2 seed lots were then screened for nodulation mutants either in the field or in greenhouse gravel beds.