To study the maternal effect of the rad201 gene, sensitivity to the lethal effect of ionizing radiation was tested during Drosophila embryogenesis. Lethality induced by gamma-rays was analyzed in progeny of reciprocally crossed females and males, homo- and heterozygous at gene mutation rad201G1 controlling radiosensitivity. No effect of the maternal genotype was detected during cleavage division. However, the effect was pronounced at the stage of postblastodermal mitotic division: progeny of heterozygous females were significantly more radioresistant than progeny of homozygous females. When studying the survival of pupae developed from larvae exposed to radiation, the maternal effect of the wild-type allele of the rad201 gene was shown to persist throughout the total larval stage. The frequency of morphoses of wings, chaetae, and abdominal tergites was also studied in adults that developed from larvae exposed to radiation at the late instar. The maternal effect was significant for abdominal morphoses resulting from the death of imaginal histoblasts. The frequency of abdominal morphoses was four times higher in progeny of females homozygous for the mutation than in progeny obtained from reciprocal crosses. Since radiation-induced damage of imaginal histoblasts is critical for pupal survival, these cells can be assumed to determine the prolonged maternal effect of the rad201 gene at the postembryonic stage of the Drosophila life cycle.