Possible carbon sources for CH4 emitted from rice paddies are organic matter applied to the fields, such as rice straw (RS), soil organic matter (SOM), and carbon supplied from rice plants (RP), such as exudates and sloughed tissues. To estimate the contribution of each carbon source to CH4 emission, a pot experiment was conducted using 13C-enriched soil sample and 13C-enriched RS as tracers. The percentage contribution of RP carbon was estimated by subtraction. When RS was applied at a rate corresponding to 6 t ha-1, the percentage contributions of RS, SOM, and RP carbon to CH4 emission throughout the period of rice growth were 42%, 18--21%, and 37--40%, respectively. The values for SOM and RP carbon for the treatment in which RS was not applied were 15--20% and 80--85%, respectively. Seasonal variations in the percentage contribution of soil organic carbon to CH4 emission were small in the range between 13% and 30% for the pots with RS and between 15% and 24% for the pots without RS. In the RS-applied treatment, RS and SOM accounted for almost 100% of the CH4 carbon early in the period of rice growth, while 65--70% of the CH4 emission in the milky stage was derived from RP carbon. ¿ 1999 American Geophysical Union