The subunit structure of phenoloxidase from the larvae of housefly, Musca domestica vicina Maquart, was investigated by urea treatment, SDS-polyacrylamide gel electrophoresis and electron microscopy. Phenoloxidase was dissociated into a single type of subunit (MW 6.5 × 104) by 5 m urea treatment. A similar subunit (MW 6.1 × 104) was also detected by SDS-polyacrylamide gel electrophoresis. Furthermore, four additional protein bands with molecular weights of 4.8 × 104, 3.5 × 104, 2.4 × 104 and 1.2 × 104 were detected by SDS-polyacrylamide gel electrophoresis by extensive incubation of phenoloxidase with SDS. A cylindrical molecular structure was deduced from the electron microscopic analysis. The outside and inside diameter and the height of phenoloxidase molecule were evaluated to be 100, 50 and 70 A, respectively.