Drosophila melanogaster larvae were reared at various constant temperatures ranging from 12° C to 32° C and adults were transferred at the usual temperature of 25° after emergence. Daily egg production and egg hatchability were measured for the whole lifetime. The adults, of the same average genotype and kept in identical conditions, showed considerable variation in reproductive capacity. For adults grown at an optimum temperature (21° or 25°) the number of offspring produced was about 2500; this number dropped below 500 when development took place either at a lower or higher temperature. Several physiological changes explain these deleterious effects: male sterility at low or high temperature, variation in ovariole number, and in rates of oogenesis and senescence. Only the male sterility was cured by the return to 25°; all the other modifications were permanent. The significance of this epigenetic load in the life strategy of the species is discussed.