The importance of vegetation in supporting methane production and emission within flooded rice fields was demonstrated. Methane emission from Louisiana, United States, rice fields was correlated to the quantity of live above-ground biomass and the rate of CO2 exchange. The quantity of belowground methane was greater in vegetated plots relative to plots maintained free of vegetation. The diurnal maximum in the rate of methane emission was coincident with the release of the most 13C-enriched methane and a maximum in transpiration rate rather than stomatal conductance, suggesting that diurnal variations in methane emission rate are linked with transpiration, in addition to temperature. Results of isotopic measurements of belowground, lacunal, and emitted methane indicate that methane is transported from rice predominantly via molecular diffusion with a small component due to transpiration-induced bulk flow. Samples of methane collected from air-filled internal spaces within the rice culm were 13C-enriched (-53.1¿0.5?) relative to emitted (-64.5¿1.0?) and belowground methane (-59¿1.0?). Reproduction of these observed 13C values with a numerical model required isotopic fractionation effects associated with transport of methane into and from rice plants. The model could not conclusively confirm rhizospheric methane oxidation. However, 13C-enriched methane was observed in the floodwater overlying the flooded soil (-44.4¿2.2?), consistent with the oxidation of substantial quantities of methane as it diffused across the soil-water interface.¿ 1997 American Geophysical Union