Although watermelon [ Citrullus lanatus (Thunb.) Matsum and Nakai] fruit have been classified as climacteric, there are reports that they exhibit cellular breakdown when exposed to ethylene gas. This study comprises an investigation of the postharvest behavior of watermelon fruit under normal conditions and in response to ethylene or propylene. Parameters measured included the effects of ethylene on ripening and respiratory activity, electrolyte leakage, cell wall ultrastructure, and cell wall hydrolases. All experiments included a comparison of the changes which occurred during both ethylene-induced breakdown and natural ripening of fruit.