Systematic observations of the atmospheric CH4 mole fraction and its carbon isotope ratio δ13CH4 have been carried out at Ny ¿lesund, Svalbard (78¿55'N, 11¿56'E) since 1991 and 1996, respectively. The CH4 and δ13CH4 showed clear seasonal cycles with respective peak-to-peak amplitudes of 48 ppb and 0.42?. By comparing the anomalies in the increase rate of the CH4 with those of δ13CH4, it is suggested that the cause of the rapid increase in the CH4 mole fraction observed at Ny ¿lesund in 1998 could be attributable to an enhanced CH4 release from wetland and biomass burning.