The growth of rice plants greatly influences CH4 emission from paddy fields through the supply of organic materials such as root exudates and sloughed tissues, the release of oxygen to the root environment, and the transfer of CH4 from the rhizosphere into the atmosphere through the aerenchyma. In the present pot experiments, the effects of the release of water-soluble organic substances from roots, the air space in roots, and the CH4-oxidizing capacity of roots on intervarietal differences in CH4 emission were examined using three Japonica type cultivars (Norin 25, Nipponbare, and Aoinokaze), which differ in morphological properties. The CH4 emission rates varied among the cultivars from mid-July (tillering stage) to the beginning of September (heading stage). Total CH4 emission throughout the rice growth period was largest for Norin 25, followed by Nipponbare, and Aoinokaze. In August, the rate of release of water-soluble organic substances from roots was largest for Norin 25. The air space in roots was also largest in Norin 25 and least in Aoinokaze. The stable carbon isotopic ratios (Δ 13C) of CH4 in roots were 3--10% higher than those in soil in August. The difference in Δ 13C values of CH4 between roots and soil was largest for Aoinokaze and smallest for Norin 25. In September, the difference in Δ 13C values of CH4 between roots and soil became small (2--3%). These findings suggest that the proportion of CH4 oxidation in the rhizosphere was largest in the cultivar which emitted the smallest amount of CH4 and that the proportion became smaller with continued plant growth. ¿ 1998 American Geophysical Union