During two successive growing seasons, methane emissions from rice fields in Italy were measured. High-precision measurements of the methane 13C/12C and D/H ratios were carried out by mass spectrometry and tunable diode laser absorption spectrometry. Significant seasonal variations were found for both δ13C and δD. The results confirm earlier observations by Bergamaschi <1997> in finding a seasonal cycle with isotopically depleted methane in the main growing season and higher values at the beginning and the end of the season during drainage of the field. The measured δ13C diurnal cycles showed a strong correlation with the methane emission rate. The isotopic composition of methane, which depended on the season, can be explained by variations of the different pathways for methane production, oxidation, and release into the atmosphere. A model based on these parameters was able to reproduce the field measurements and indicate the principal causes of observed fluctuations in the isotopic methane composition.