The problem of genetic effects from low dose level radiation is a very important one. We have tested the linear relationship between dose and mutation rates of Drosophila at doses as low as 8 r. The material used was the wild Canton-S strain which were isogenized every four months. Wild males were stocked for one week after their emergence, and irradiated with X-rays. Sex-linked recessive lethals were detected by M-5 method, and about 530, 000 X-chromosomes including the control were examined. The straignt line obtained at the high dose level was in good accord with that obtained by Spencer and Stern (1948). And the curve obtained at high dose level was found to be applicable at the low dose level as well. Among the 26, 897 males treated, there were 25 cases which produced two to fifteen lethals. How to treat the simultaneous occurence of two or more lethals in a set of twenty tested chromosomes originated from one treated male is an important and troublesome problem. However, sufficient evidence is available to conclude that the linear relationship between mutation rates and radiation dose can be applied to low doses down to 8r, by using the sex-linked recessive lethal mutaions in Drosophila, one of the most reliable indicators of mutation induction.