We reported previously that low-dose X irradiation of DNA repair-proficient immature sperm of wild-type Drosophila melanogaster at a low dose rate (50 mGy/min) resulted in a mutation frequency that was lower than that in the sham-irradiated group. Therefore, a U-shaped dose–response relationship was suggested. Here we show that the dose–response curve is actually U-shaped by carrying out a large-scale sex-linked recessive lethal assay using Drosophila. No reduction of the mutation frequency was observed in a strain mutant for the nucleotide excision repair gene mei-9a (Drosophila homologue of human XPF). Introduction of a chromosome fragment containing mei-9 into the mei-9a mutant strain restored the reduction of the mutation frequency in the low-dose-irradiated group. These results showed that DNA repair was responsible for the U-shaped dose–response relationship in Drosophila.