In the silkworm, Bombyx mori, diapause occurs at a specific embryonic stage, i.e. after formation of the germ band with cephalic lobes and telson and sequential mesoderm segmentation. As long as the eggs are incubated at 25 ยฐ C, cell divisions and morphological development of the embryos cease. To examine changes in percentage of embryonic cells in the G1, S and G2 phases during embryogenesis, nuclear fractions were isolated from embryos, stained with propidium iodide and then subjected to flow cytometric analysis. The percentages of embryonic cells in G1, S and G2 were 10, 35 and 55%, respectively, at the stage of formation of cephalic lobes, whilst 98% of cells were in G~ at diapause stage. After termination of diapause by acclimation at 5 ยฐ C or by a combination of chilling and HC1, cell division resumed in the embryos. During this period, the cells rapidly entered S phase through G~ from G2, suggesting that their G~ phase was short. In eggs in which diapause was averted by HCl-treatment after incubation at 25 ยฐ C for 20 h after oviposition, embryonic development proceeded continuously for 9.5 days at 25 ยฐ C until hatching. Along with this development, the Ga fraction increased to levels of about 90%. These results indicate that embryonic cells are arrested in G2 at diapause and suggest that, concomitant with further embryonic development, cell cycles become slower in proportion to an increasing length of Gt. Finally, most of the cells may be arrested in G~ while there is only a small fraction of cells continuously cycling.