Effective combinations of peanut (Arachis hypogaea L.) cultivars and Bradyrhizobium strains were determined under field conditions in NE Thailand in experiments conducted in 1985 and 1986. The experiments measured yields and N₂ fixation (N-15 dilution) responses of peanut cultivars Tainan 9, Moket and KAC431 to inoculation with inoculants composed of Bradyrhizobium strains TH205, 32HI or NC92 or mixed cultures of bacteria. Midseason parameters of nodule number and weight and acetylene reduction levels were also determined. Peanut yields varied between years and locations. Cultivar Tainan 9 was shown to be the cultivar with the best average yield for the locations tested. Inoculation with bradyrhizobia resulted in increased seed yields and amounts of N in the plants in most cases. Inoculated peanuts received an average of 71 kg N/ha from fixation; uninoculated peanuts received an average of 62 kg N/ha. Of this N, 55 to 77% remained in the stover. Midseason measurements of plant performance were of minimal value for predicting the relative performance of the three cultivars.