The glycogen content of the eggs of silkwormBombyx mori L. was analysed at intervals during the course of embryonic development and diapause. Changes in the glycogen levels were also examined in acid treated eggs in which diapause has been broken. These studies showed significant differences in glycogen utilisation pattern in the diapause and non-diapause eggs. The activity of the enzyme glycogen phosphorylase was also found to exhibit different patterns in the two types of eggs. Our studies indicate that this enzyme plays an important role in the metabolic changes which occur during diapause in silkworm eggs.