Abstract Selected stages of newly laid inseminated eggs of Drosophila melanogaster were X-rayed with 500 R. The investigated phase of development last about 16 min. During this period the egg nucleus goes through the late stages of meiosis and forms the maternal pronucleus. Simultaneously the sperm head transforms into the paternal pronucleus. By using a new experimental arrangement it was possible to register in the same experiment three types of mortality as well as the recessive lethals (including orthodox and Y-suppressed lethals) in the maternal and paternal X-chromosome separately. The radiosensitivity for early and late embryonic, as well as for postembryonic mortality, changes during the development. For each of the three types of mortality this change is correlated in a similar way to the nuclear division cycle. Irradiation of anaphase/telophase of the second meiotic division gave maximum mortalities. Pronuclei on the other hand are least sensitive. Concerning the induction of recessive lethals in the maternal X-chromosome, all meiotic and pronucleus stages so far investigated are equally sensitive (average rate 2.9%). Contrary to this, the radiosensitivity of the paternal X-chromosome changes significantly during the transformation of the sperm head into the paternal pronucleus. The average rate of sex-linked lethals of sperms irradiated in the egg 1 to 10 min after egg deposition was 4.75%. This rate is significantly higher than (a) the average rate of 2.25% determined for the older stages of the sperm (irradiated 10 to 16 min after egg deposition) and (b) the rate of 2.9% mentioned above. For each irradiated stage the lethal factors found in both X-chromosomes were added to give the total rates of lethals. If these are compared with the corresponding total mortalities it becomes evident that the induction of lethal factors and the induction of mortality are correlated in a different way to the nuclear division cycle. For both radiation effects, however, the pronucleus stages are least sensitive. The question whether the radiation-induced mortality is caused mainly by chromosomal changes cannot be answered yet, since the specific radiation effects in the paternal and the maternal genome, respectively, are not known.