In diapausing eggs of the silkworm Bombyx mori, embryonic cells are arrested at G2 phase. The ability to undertake cell division is resumed in the course of diapause termination caused by such a treatment as acclimation to 5 degrees C. As an initial trial to investigate the relationship between diapause and embryonic cell cycling, we have cloned and sequenced two Bombyx cDNAs encoding two distinct cdc2-related Ser/Thr protein kinases. One (Bm cdc2) encoded a 37.0 kDa protein which had all of the domains characteristic of other Cdc2 kinase. The other (Bcdrk) encoded a 45.1 kDa protein that was most similar to Drosophila and human cdc2-related protein kinases (Dcdrk protein and PISSRLE kinase). Northern blot analysis was carried out to examine levels of Bm cdc2 and Bcdrk mRNA during embryogenesis of non-diapause eggs. The result demonstrated that the mRNA level of Bm cdc2 appeared to correspond to the activity of nuclear/cellular division in non-diapause eggs, and that the developmental profile in the level of Bcdrk mRNA was somewhat different from that of Bm cdc2 mRNA.