A new grandchildless, maternal-effect temperature-sensitive mutant of Drosophila melanogaster, gs(2) M, was isolated in our laboratory. At 28.5 degrees C, homozygous gs(2) M/gs(2) M females lay a normal number of eggs, but about 20% of them fail to hatch and about 40% die just after hatching. The remaining embryos, which pass through this critical stage, complete their development normally, but some of them are devoid of pole cells and thus produce agametic adults. The death of embryos is maternally determined and the hatching probability of an embryo does not depend on its own genotype. The influence of several factors on the phenotypic expression of the new mutant, e.g., age of the females, temperature and number of generations under homozygous condition, is described. Mutants of the type represented here could be useful for further analysis of the establishment of the germ line in Drosophila.