(1) Changes in consentration of the blood constituents of the silkworms fed with the mulberry leaves of different stages of development were investigated. (2) Among several constituents in the blood investigated, concentration of total and protein nitrogen, and total and inorganic phosphates show a great variation according to the difference of mulberry leaves fed. These constituents show high percentage in the blood of the silkworms fed with the young mulberry leaves which are rich in these constituents. (3) The concentration of calsium, magnesium and chlorides in the blood show a little variation, and are a little higher in the blood of the silkworms fed with the mulberry leaves in full growth, which are very rich in these constitutions. (4) Most of the constituents of blood vary generally in concentration according to these of the mulberry leaves fed.