Rice paddies contribute significantly to the atmospheric burden of CH4 but may also sequester atmospheric CO2. Previous studies based on putative relationships between net CO2 exchange and CH4 emissions have concluded that globally significant amounts of carbon can be stored in rice paddy soils. However, the annual ratio of CH4 emissions to net CO2 exchange has not previously been measured. We simultaneously measured the net exchange of CO2 (F CO2) by eddy covariance and the CH4 emission rate (F CH4) using a combination of a flux gradient technique and weekly chamber sampling. During rice growth, F CH4 was 1.9% to 2.4% of net carbon uptake (mole per mole). F CO2 closely followed biomass accumulation. In contrast, F CH4 increased during vegetative rice growth and decreased over the ripening and reproductive phases of rice growth, suggesting that the plants release substrate for methanogenesis early in the season. CH4 emissions represented 4.8% to 5.6% of the net CO2 uptake when summed over an entire year (including a 20-week period over which the field was unplanted and flooded). Assuming harvested rice is remineralized within 1 year, the remaining 0.67 t C ha-1 that was sequestered by the paddy potentially offsets the radiative forcing of the emitted CH4 by 26% to 31%. The ratio of F CH4 to F CO2 varied widely over the course of a year depending on management practices in a specific field. The results reported here emphasize the importance of year-round measurements to obtain a reliable estimate of CH4/CO2 exchange stoichiometry.