The time in hours from laying to hatching at 17°C. has been measured for eggs from four inbred lines ofDrosophila subobscura, and also for F1eggs, and for eggs obtained by crossing F1hybrids to unrelated inbred lines and to unrelated F1hybrids.Eggs laid by females of a given genotype have a characteristic rate of development, which, with one exception, is not greatly influenced by the genotype of the male parent. F1hybrid females laid eggs which were more uniform in hatching time than eggs laid by inbred females, and which developed more rapidly than eggs laid by three of the four kinds of inbred female studied. The slowest-developing eggs were laid by females from theNFSinbred line, a line originally selected for slow development from egg to adult.The male parent, and hence the zygotic nucleus, had an appreciable effect on the rate of egg development only for eggs laid byNFSfemales; these eggs developed more rapidly if fertilized by sperm from an unrelated male.