Silkworm larvae at the fifth instar were made to abstain from mulberry leaves after feeding for 0 to 5 days. Control larvae matured after feeding for 6 days, spun cocoons and went into metamorphosis. Larvae fed for only 0 to 2 days died mostly before pupation and some before emergence of moths. Effects of the starvation stress on the egg formation which takes place during pupal life were investigated. 1) The number of mature eggs was most severely affected to reduce by starvation stress as compared with size or weight. The longer the fasting, the more reduced the egg number. By fasting the last one day of the fifth instar, eggs became rather slender with no detectable changes in weight. The weight of an egg reduced only in those made by animals starved after feeding for the early three days in the fifth instar. 2) Contents of yolk protein tended to decrease with the shortening of feeding period. The amounts of the egg specific ESP protein, one of major components of yolk protein synthesized by the ovary, decreased significantly by starvation. While those of the vitellogenin, transferred from pupal haemolymph into the oocytes, were not reduced at all. It may be inferred that there exist two different regulatory mechanisms in the oogenesis of silkworm. In response to unusual environment, a regulation process concerning the number of eggs to be matured comes into action at first, then against unsaturated shortages acts an egg size controlling mechanism.