1. Larval and pupal phenoloxidase of the housefly, Musca domestica L. resembled each other in some enzymatic properties, such as optimal pH, pH stability, optimal temperature and thermal stability. 2. The pI values of both enzymes were 4.85, as determined by isoelectric focusing. The molecular weights of two enzymes had a similar value (320,000 and 330,000 for larval and pupal phenoloxidase, respectively) and the molecular weights of these subunits had the same value of 60,000. 3. The aminio acid compositions of two enzymes were very similar. N-terminal amino acid sequences of larval and pupal enzymes were Glu-Glu-Ala-Thr-Val-Val-Pro-Asp-Gly- and Ala-Thr-Val-Val-Pro-Asp-Gly-Tyr-Phe-Met-, respectively, and the residues 3-9 (larval phenoloxidase) were in agreement with the residues 1-7 (pupal phenoloxidase).