Soybean [Glycine max (L.) Merr. cv. Bay] seeds were irradiated with X-rays (25kR) and the M2 progeny was screened for changes in the fatty acid composition of seed oil. X-ray irradiation remarkably increased the variability of the fatty acid composition in the oil of the Bay cultivar. A mutant in which linolenic acid accounted for 18.4 per cent of the total oil cornpared with 9.4 per cent in the Bay cultivar was identified among 2006 M2 plants. The M3 generation of the mutant also showed a linolenic acid content approximately two times higher than that of the original variety.