Abstract: The tolerance levels of citrus trees to different degrees of drought and flooding was studied by using twenty-three species and varieties. Rough lemon, trifoliate orange, lemon and Cleopatra mandarin were evaluated as the most flooding tolerant; Yuzu, ‘Kawano’ natsudaidai, Kinukawa-mikan, ‘Tanaka’ navel orange and Ponkan as medium; and Hana-daidai, Kosuito, Naruto-mikan and Natsudaidai as least tolerant. Trifoliate orange, Yuzu, Shiikuwasha, Hana-daidai, ‘Tanaka’ navel orange, Natsudaidai, Shuto, Yamabuki , Ponkan and ‘Shiranui’ were evaluated as the most drought tolerant; Cleopatra mandarin, Naruto-mikan and Murcot as medium and Rough lemon, lemon, Ban-okan and ‘Kawano’ natsudaidai as drought intolerant. The effect of flooding and drought on 1-aminocyclopropane-1-carboxylic acid (ACC) content and ethylene production in the roots of these several species and varieties was determined. The ACC content and ethylene production in the roots increased more rapidly in sensitive species in response to water stress. Under flooding conditions, roots maintained higher ethylene production levels than control roots. Sensitive species showed a little greater levels in ethylene production than the tolerant ones.