Treatment with ozone also induced the activity of PPO and POD (Figs. 6 and 7) in papaya fruit stored at 25◦C. PPO activity in 5.0 µL L−1ozone-treated papaya fruit reached its highest value(180 UmL−1enzyme extract) after 10 days in ambient storage. PPO activity in fruit exposed to 1.5 µL L−1, 2.5 µL L−1, 3.5 µL L−1 and 5.0 µL L−1ozone were 2.2 fold, 2.4 fold, 2.6 fold and 3.4 fold, respectively, higher than the untreated fruit at day 10 (Fig. 6). The effect of ozone on PPO activity was significant. The activity of this enzyme in all ozone treated fruit was higher than in the untreated fruit stored at ambient conditions. However, after day 10, PPO activity in the ozone treated fruits declined and on day 14 was similar to that of untreated fruit. This rapid decline of enzyme activity after day 10occurred as fruit became senescent and over-ripe.