Isolation and characterization of ribosome from the midgut of Bombyx mori, and their capacity for in vivo protein synthesis at different stages of development were studied. Conventional procedures for ribosome isolation were ineffective when applied to Bombyx mori. The preparation of ribosomes from this material depended upon the elimination of Mg ion from the homogenizing medium, the presence of a moderately high KC1 concentration in the homogenizing medium, and the addition of Triton X-100 rather than deoxycholate to post-mitochondrial fraction. The ribosomes prepared in this manner had a RNA content of 55 per cent, A260: A235 ratio of 1. 9, with absorption maxima and minima at 260 and 235 nm, respectively, and were given 80S by sucrose density gradient centrifugation analysis. The ribosome had a particle of diameter of 200 A by electron microscopic observation. The ribosomes prepared in this manner had a capacity for in vitro protein synthesis. Addition of this ribosome to E. coli cell-free system stimulated amino acid incorporation into protein. By the use of in vitro protein synthesizing system with ribosomal and supernatant fractions from different stages of the development (glutoneous and maturation stages), their capacity for in vitro protein synthesis was studied at different stages of the development. The level of amino acid incorporation exhibited higher by ribosomal and supernatant fractions from maturation stage than that from glutoneous stage. Endogenous capacity for protein synthesis of ribosomal fraction from maturation stage exhibited higher than that from glutoneous stage, and also it is true of endogenous capacity for protein synthesis of supernatant fraction from maturation stage. The activity of protein synthesis is higher at the stage of maturation than at the stage of feeding. Results obtained by the use of in vitro protein synthesizing system suggest that the nature of ribosomal and supernatant faction from the stage of maturation is different from that of the glutoneous stage and control of protein synthesis takes place at each of the levels of ribosome and supernatant.