Abstract Radiation-induced nondisjunction in Drosophila melanogaster females usually-possibly invariably-involves the participation of chromosomes other than the pair in which the numerical aberration is noted, with one of the two acrocentric pairs frequently being involved in the assortative error of the other. Nearly one-half of all diplo-X eggs produced following the irradiation of immature oocytes of females having free X's are found to be nullo-4, and, in agreement with earlier reports9,11, about one-fourth of all nullo-X eggs are diplo-4. The incidence of structural alterations is markedly higher in chromosomes involved in nondisjunctions than in those recovered from normal segregations, with the structural changes being those expected from interchange between X and fourth chromosomes where only one of the two interchange products (a “half-translocation”) is recovered. X chromosomes may acquire an arm of chromosome 4, and fourth chromosomes may lose the marker from the left arm, as if the short, heterochromatic right arm of the X had been substituted. Homozygosis of markers near the centromere of the X chromosome shows that nearly all failures of segregation must occur at division I. While the data do not require that there be some division II nondisjunction, neither do they categorically deny the possiblility of its occurring at a very low level. The findings are as expected on the model of heterologous conjunction via chromatid interchange as the major and perhaps exclusive cause of radiation-induced nondisjunction.