Acrylamide gel thin layer electrophoresis was used to study the development and distribution of blood proteins in the silkworm, Bombyx mori. More than twenty bands, the tremendous number of which has not yet been found so far, were demonstrated in the fifth larval instar and pupa. In the first and the early second larval instar one specific protein band, no. 18, was detected, and the band disappeared in the middle stage of the second instar. Two characteristic bands, nos. 11 and 21, were found to be predominating in younger instars. Concentration of the band 11 was gradually diminished and finally disappeared in the fourth instar, while the band 21 was still remained until the time of emergence although it was reduced in amount. Both number of bands and total amount of protein constituents were more remarkably and rapidly increased during the last fifth larval instar than any other stages of life cycle. The most striking change in the fifth instar was the appearance of bands 23 and 30. The second apparent change was the increase in concentration of proteins 19 and 26. The two distinct bands which might be called as larval female proteins (FL-1 and FL-2) were found in the middle and later stages of the fifth instar. After pupation slight change in electrophoretic pattern, compared with that of the late fifth instar, was observed except notably reduced amount of band 26 and the conversion of "female specific" proteins. It is of interest that the larval female proteins disappeared completely just after pupation, and in turn a faster moving band was newly detected in female pupa which was absent in the male. So this new band stands for a pupal female protein (FP). As the pupal stage progressed, there was a general indication of decrease in the amount of major pupal proteins, followed by a sudden severe reduction on the day before emergence to remain in trace amounts. On the contrary, the band 34 increased rapidly in concentration at the time of emergence and served as the main fraction of the adult moth. This simultaneous changes of blood proteins during life cycle of the silkworm provided a detailed example of the logical assumption that the biochemical development of proteins precedes the more obvious morphological changes in metamorphosis.