The characteristics of the mus(2)201G1 mutant controlling the mutagensensitivity of Drosophila were studied. The data obtained on the survival of flies affected by MMS of UV light during the larval stage of the development show that this mutation determines the extremally high sensitivity of early and late larvae of Drosophila to the lethal action of both mutagens. The primary embryonic cell cultures were used to estimate the influence of mus(2)201G1 locus on the repair of radiation-induced genetic damages. The rate of excision of UV-induced pyrimidine dimers in the DNA was examined and shown to be significantly lower in mutant cells, as compared with those of the while type files. Incomplete elimination of photodimers is, apparently, due to the defect of the UV-specific endonuclease, since the extracts of mutant cells reveal the 4-5 fold decrease in the corresponding enzymatic activity.