Occurrence of protyrosinase in the prepupae of housefly, Musca vicina Maquart, and its activation were described. The prepupae possessing no appreciable tyrosinase activity could be separated from the other aged pupae by putting them into water. Homogenate prepared from the prepupae contains protyrosinase and has no activation system for the proenzyme. As has been reported by Ohnishi2) a certain activator occurred naturally in the aged pupae apparently activates the protyrosinase in vitro. However, contrary to Ohnishi’s results3) it was found that this protyrosinase can be activated by the treatment with sodium dodecyl sulfate in vitro.