However, little information is currently available about ozone-induced oxidation of ethylene, fruit quality and changes in the activities of the phenolic enzymes, phenylalanine ammonialyase (PAL), peroxidase (POD) and polyphenol oxidase (PPO), in papaya fruit. These enzymes are very important in plant disease resistance. They are involved in the formation of lignin and phytoalexins, which are related to plant disease development (Grahamand Graham, 1996). This paper reports the effect of ozone exposure on peel colour, respiration rate, ethylene production and PAL, PPO and POD activities in papaya fruit stored at ambient temperature.