After treatment of mature spermatozoa of Phryne cincta egg mortality rates were determined and chromosomal mutation rates by examination of the polytene chromosomes of the F1 progeny.--A comparison of the results with similar studies made on Drosophila melanogaster yielded a two- to threefold higher sensitivity in Phryne cincta. Evaluation of data concerning DNA amounts, heterochromatin contents, distribution of breaks, repair capacities in two-break sites and oxygen concentrations in mature spermatozoa revealed no contradiction to the hypothesis that the greater sensitivity of Phryne cincta is mainly due to a high degree of spiralization of the paternal pronucleus chromosomes.