The fruit of stony hard peach cultivars 'Odoroki', 'Manami' and 'Yumyeong' usually remain hard and do not synthesize ethylene even after their ground color changes, sugar contents increase, and acidity decreases. Our treatment of such fruit with l, 000 ppm ethylene for three consecutive days at 25°C effectively softened them. Ethylene treatment of 'Yumyeong' fruit with l, 000 ppm for three consecutive days at 5°C did not cause them to soften. Treatment at 15°C softened the fruit, which started one day later than those treated at 25°C. All these treatments did not induce ethylene synthesis of the fruit. When the treatment of 'Yumyeong' fruit with l, 000ppm at 25°C was ceased after the first day, the fruit stopped softening and showed no ethylene synthesis. Treatment of 'Yumyeong' fruit with 1-aminocyclopropane-1-carboxylic acid (ACC), which is the immediate precursor of ethylene, stimulated the fruit to synthesize ethylene and soften. The results show that the fruit of stony hard peach cultivars soften in the presence of ethylene, but they cannot produce endogenous ethylene because of the lack of or low activities of ACC synthase. This suggests that the possibility exists of controlling flesh firmness by changing the duration of ethylene treatment.