A mutant line, M23, of soybean [Glycine max (L.) Merr.] was found to have two fold increases in oleic acid content in the seed oil compared with the original variety, Bay. Our objective was to determine the inheritance of the high oleic acid content in this mutant. Reciprocal crosses were made between M23 and Bay. There were no maternal and cytoplasmic effects for oleic acid content. The F1 seeds and F1 plants were significantly different from either parents or the midparent value, indicating partial dominance of oleic acid content in these crosses. The oleic acid content segregated in the F2 seeds and F2 plants in a trimodal pattern with normal, intermediate and high classes, satisfactorily fitting a 1∶2∶1 ratio. The seeds of a backcross between M23 and F1 segregated into intermediate and high classes in a ratio of 1∶1. These results indicated that oleic acid content was controlled by two alleles at a single locus with a partial dominant effect. Thus, the allele in M23 was designated ol and the genotypes of M23 and Bay were determined to be olol and 0l0l, respectively. The oleic acid contents of the F2 seeds and F2 plants were inversely related with the linoleic acid content which segregated in a trimodal pattern with normal, intermediate and low classes in a 1∶2∶1 ratio. Thus, it was assumed that the low linoleic acid content in M23 was also controlled by the ol alleles. Because a diet with high oleic acid content reduces the content of low density lipoprotein cholesterol in blood plasma, the mutant allele, ol, would be useful in improving soybean cultivars for high oleic acid content.