The content of ribosomes was high at the initial stage of the pupa of Bombyx mori, but it declined to a minimum level at the mid-pupal stage. The activities of isolated ribosomes to incorporate amino acids into protein showed a similar variation to that of the ribosome content. The bulk RNA extracted from the pupae at the middle stage showed heterogeneous fractions when analyzed by sucrose density gradient centrifugation. During the latter half of the pupal period, the ribosome content increased and the isolated ribosomes again showed high activities to incorporate amino acids. These facts indicate that changes in nature as well as in content of ribosomes may play important roles in the control of protein synthesis during adult development of Bombyx mori.