The frequency of occurence of white - yellow stripes on plant leaves,developed from irradiated maize seeds containing lemon - white gene (Lw1/lw1), was used as a criterion in comparing the mutagenic effects of high LET heavy ion beam and low LET radiation. Dose - response curves for 20Ne,40Ar,56Fe ion beam, and 60Co gamma rays were linear. The RBE for neon, argon and iron beam, when compared with 60Co gamma rays, were 2. 02, 8. 31 , and 12. 45, respectively. Lemon -white gene nmtalion was observed in inbred plants in the second generation. Electron microscope observation showed that both the structure and the number of chloroplasts in mesophy Is of white -yellow stripes were significantly altered.