Two different populations of gametophytes of L. japonica were used to study the biological effect of Co60γ－radiations and the different radiosensitivity. They were derived from different lines of sporophyte after four years of inbreeding. One population came from a mature sporophyte characterized with no margin, while the other population from one with very narrow margin. It was observed that the biological effect of Co60γ-radiations was comparable to that of X-radiations. High doses of radiations killed many gametophytes. The gametophytes derived from the sporophyte without margin were found to be much more sensitive to Co60γ-radiations than those from the sporophyte with very narrow margin. It was suspected that the difference in the observed radiosensitivity was due to differences in heredity.