Very young sporophytes of Laminaria were used in this experiment. From the present experiment the following observations were made: 1. The high dose of 10,000 r of Co60γ-radiations did not cause any immediate death of the sporophyte cells. Death occurred as usual in the next day after γ－irradiation. 2. The young sporophytes which are diploids seem to possess higher radio-resistance to Co60γ-radiations than do the female gametophytes which are haploids. 3. Both the lower and higher doses of Co60γ-radiations all showed some stimulating effects on the young sporophytes. The lower doses seem to have quickened the rate of cell division, while the higher dose of 10,000 r seems to have made the cell swell in the cytoplasm and this eventually caused the death of the sporophytes.