In adult caged house flies, maintained on a complete diet of powdered whole milk, cane sugar and water, the median longevity, maximal longevity, and percentage that exhibit complete wing retention at death are increased as the number of individuals per cage is correspondingly decreased from 240 to 10 flies per cage. These effects are more marked for the shorter-lived males than for the females. Female longevities at all population densities are more significantly reduced by a restricted diet of sugar and water only. Under conditions of individual confinement, the life expectancy of the female house fly is no greater than that of the male. These results are in agreement with earlier suggestions that increased physical activity including interaction among individuals in a restricted population decreases longevity.