The frequency of reicprocal translocation damage in males of D. melanogaster irradiated with X-rays was observed in this study. The frequencies were checked at four periods with two days intervals and duration spermatogenesis after irradiation. (1) Modification in the percentage of the reciprocal translocation damage were not obtained at interval after irradiated with 500r and 1500r of X-rays respectively. (2) In two experimental groups irradiated with 50-0r and 1500r of X-rays, the frequencies showing in the spermatogenesis were 0.50%(500r), 3.85%(1500r) in mature sperm, and 1.59%, 8.10% in the spermatocyte. (3) The frequency of reciprocal translocation between the Y and 3 rd chromosomes was the highest, but in accordance with dosage increase that of 2nd and 3rd chromosomes relatively increased from 9.34 % to 30.49% while decreased from 68.75% to 46.80% in the group of the Y and 3 rd chromosomes. (4) It was supposed that these modifications of the frequency were due to heavy damage of the 2nd chromosomes than other chromosomes in accordance with dosage increase. (5) Spontaneous reciprocal translocations involving the Y, 2nd and 3rd chormosomes was 0.23%.