1-Aminocyclopropane-1-carboxylate (ACC) oxidase, the enzyme that catalyzes the conversion of ACC to ethylene, the final step of ethylene biosynthesis was extracted from wounded mesocarp tissue of winter squash (Cucurbita maxima Duch. v. Ebisu) fruit. The enzyme was characterized with respect to temperature optima, thermostability, stability in the presence f selected metal ions and alkylating agents, and Km value for ACC. ACC oxidase requires Fe2+ as a co-factor and maximum activity was achieved using Fe2+ at 20uM in the reaction mixture. The enzyme was activated by C02 and inclusion of C02 in the reaction mixture increased the apparent Km value of the enzyme with respect to ACC. The enzyme exhibited apparent Km values for ACC of 147uM in air and 454 uM in the presence of 5 % C02. ACC oxidase was partially inactivated by ACC during its catalytic action. The enzyme had maximum activity at 30℃ and its activity was almost completely lost m the presence of Ag+, C02+, Cu2+ and Zn2+ ions. The alkylating agents, iodoacetamide and iodoacetic acid partially inhibited and almost completely abolished ACC oxidase activity respectively, thereby suggesting requirement of sulfhydryl groups for ACC oxidase activity. Following excision of the mesocarp tissue, the increase in in vivo ACC oxidase activity was accompanied by a concomitant increase in in vitro ACC oxidase activity. The results support and extend previous observations and indicate that the solubilized enzyme is indeed ACC oxidase and resembles the physiological ACC oxidase in several aspects.