Abstract The activity of ribosomal RNA (rRNA) genes as judged by nucleolar size and rRNA synthesis has been shown to depend upon the phase of diapause in the eggs of Bombyx mori. In the present study, we found that nucleolar size in diapausing eggs was enlarged at a very early stage during cold treatment, a procedure necessary for the termination of diapause. In contrast, the intrinsic capacity of ribosomal RNA synthesis in the chilled eggs, as examined at 25°C by radioactive precursor incorporation into rRNA, increased much later, in parallel with the break of diapause. The early phase of cold treatment is the period when the eggs undergo some important changes (the so-called diapause development), preparing for diapause termination. Thus we infer that the above mentioned increase in nucleolar size may be one of the features of diapause development.