We reported that the hemolymph of the a80 strain of the silkworm, Bombyx mori, included three isoformes of prophenoloxidase, a proenzyme of phenoloxidase. However, their heterogeneity in strains other that a80 has not been investigated. On polyacrylamide gel electorophoresis followed by activity straining for this protein, we discoveres that two pro-PO isoformes were revealed in the larval hemolymph of some strains, two strains have three isofarms in their hemolymph, and only hemolymph od a481 strain contain one isofarm among silkworm strains we tested. In this paper, we confirmed the poilymorphism of prophenoloxidase.