The total free amino acid levels declined from the early-fourth instar to the mid-fifth instar and were elevated during the late-fifth instar in the metamorphosing silkworm,Bombyx mori. The activity levels of aspartate and alanine aminotransferases showed an increase in the silk gland while they decreased in the central nervous system, muscle and hemolymph. The molting period was, however, characterized by low activity levels in all the tissues studied. The total and soluble protein levels increased continuously during metamorphosis. The results are correlated with active protein synthesis and silk production in the developingBombyx mori.