Dry seeds of soybean [Glycine max (L.) Merr. var. Bay] were irradiated by X-rays (21.4 kR) and the M2 generations were examined for the oleic acid content in their oil. The genetic variability of oleic acid content in the oil of the Bay variety was significantly increased by the X-ray treatment when compared with the Bay control. A mutant, designated as M23, was selected from 2747 M2 plants, having an oleic acid content of 46.1%, twice as much as that in the normal Bay variety. A distinct inverse relationship was observed between the oleic and linoleic acid contents in M23. Mutant M23 isolated from the M2 generations was confirmed to be always associated with a high oleic acid content under different environmental conditions in the M3 generations.