The pink bollworm, Pectinophora gossypiella (Saunders), was reared on purified casein diets containing 0.25, 0.50, 1.5, 3.0, 4.0, 5.0 and 6.0% fat, respectively. The fat was supplied to the diets in the form of corn oil. Life history records indicated the incidence of diapause increased with the fat content of the diet. Diets containing 0.25% fat apparently did not satisfy the minimum requirements of the pink bollworm since many of the individuals produced were partially devoid of body scales, had crimped, twisted wings and were not able completely to escape the pupal case. An increase in fat content above 0.5% resulted in a concomitant increase in the duration of the immature stages and a reduction in pupal weight.