By means of thin layer electrophoresis in agar gel, the zymograms of blood esterase of 161 silkworm strains was studied. Four fundamental types of esterase were found, and crossing experiments led to the conclusion that they are controlled by codominant alleles, Bes^A, Bes^B, Bes^C, and Bes^O. About 70% of the Japanese, Chinese and European races investigated belong to A type and 20% to O type, while B type was found only in Chinese race.