An experimentally-produced hybrid between red and green plants of Beta palonga and the segregating population in F 2 showed more resistance to acute X-irradiation of seed than did their parents, as measured by seed germination, plant height, abnormal plant growth, chromosomal abnormality during meiosis and pollen grain sterility. In every measurement the F 2 plants were more sensitive than the F 1 hybrid. The radiosensitivity could not be interpreted in terms of average interphase chromosome volume. The resistance of the hybrid is suggested to be due to factors associated with heterozygosity and heterosis, although this has not been definitely established.