We have made measurements of the methane (CH4) flux and &dgr;13C value in CH4 from rice paddies in Ryugasaki, Japan. This study is the first we are aware of in which a significant change in the &dgr;13 C signature of emitted CH4 has been documented over the rice growing season. Nutrient treatments studied were of two kinds: compound mineral fertilizer either with or without rice straw from the previous growing season incorporated into the inorganic fertilizer. The calculated annual emission rates during the 1990 growing season were 43.1 g/m2 (straw) and 40.6 g/m2 (no straw) for the two treatments. In both treatments, CH4 started out relatively enriched in 13C, became lighter in 13C, and then became more enriched again during the latter part of rice growth. The 1991 growing season showed a lower integrated flux in both nutrient treatments than for 1990 but plots of the fluxes versus time had the same general shape as the flux curves in 1990 and a similar although less pronounces trend in &dgr;13CH4 signal. Seasonal changes in &dgr;13C are probably related to changes in CH4 production and oxidation and plant-mediated transport. The likelihood of each process occurring and its effect on 13C values is discussed. The range of &dgr;13CH4 values from seasonal effects was ~12%0 in 1991 for both nutrient treatments. The &dgr;13CH4 range for 1991 was ~10%0 (straw) and ~5%0 (no straw). Our data indicate that when using flux-weighted isotopic signatures to put constraints on the tropospheric CH4 budget, attention should be paid to seasonal changes in isotopic signatures from rice paddy CH4 in a manner similar to that previously suggested from measurements in natural wetlands.