Genetic control of somatic mutagen sensitivity was studied in a strain of Drosophila melanogaster containing mutations in the second chromosome. The method used allowed to identify the determinants of different kind of sensitivity in the same experiment. It has been shown that the second chromosome of the strain analysed carries two independent mutations, one of them determining supersensitivity of Drosophila larvae to ionizing radiation, the other--to methyl methane-sulfonate. Mutant loci have been located at 59,9 +/- 1,1 and 73,8 +/- 2,5 units of the chromosome genetic map. They have been called rad(2)201G1 and mus(2)201G1, respectively.