DURING the course of a study on the biochemical changes accompanying ageing in the adult house-fly, the attention of the senior author was directed to the differential longevities of the male and female members of the common house-fly, M. domestica L. Paradoxically enough, a search of the literature revealed only scattered and inconclusive information on the longevity of this species under controlled laboratory conditions. Several authors have been cited by Roubaud1 to the effect that the longevity of the house-fly can be extended to as much as 90 days by exposure to temperatures of 15° C. (59° F.); West, on the other hand2, states in no convincing terms that the longevity of the house-fly varies from two to three weeks, at most, during the summer and up to three months during the winter months. A side study was therefore undertaken to obtain reliable vital statistics for the male and female members of this species, bred, reared and maintained under standardized laboratory conditions. This report will summarize briefly one phase of this study to be published, in extenso, at a later date.