Arctic wetland soils are substantial natural sources of the climate relevant trace gas methane. Microbial methane oxidation is an important control of methane emissions from these environments. We investigated the abundance and activity of Methane Oxidizing Bacteria (MOB) in two waterlogged soils (a Fibristel and a Historthel) from different tundra sites in Northern Siberia. The size of MOB populations was quantified by group-specific phospholipid fatty acids for type I and type II methane oxidizers. To determine the active part of the MOB population, we labelled soil samples with 13CH4 and analysed the 13C content of phospholipid fatty acids. The temperature dependence of methane oxidation was measured in batch incubations. Fatty acids specific for type I MOB dominated strongly over those for type II organisms throughout the profiles of both tundra soils. Up to 8 % of all fatty acids were specific for MOB, indicating a considerable contribution of methane oxidizers to the total bacterial biomass. The stable isotope experiments showed that type I organisms were not only the most abundant but also the most active MOB in both tundra soils. More than 85 % of 13C incorporated into phospholipid fatty acids could be found in fatty acids typical for the type I group of MOB. The concentration of MOB fatty acids decreased with depth in both waterlogged soils, correlating well with depth distribution of potential activities. However, in subsoil horizons without measurable activities, MOB-fatty acids were still present, indicating the occurrence of inactive cells or spores that may be reactivated under changing environmental conditions. The Fibristel had a higher organic carbon content of 37 % and contained 20 times more fatty acids specific for MOB (16.3 nmole cm-3) than the Historthel with 18 % organic carbon and 0.8 nmole cm-3 MOB specific fatty acids. MOB communities in both soils showed maximum activities around 28°C, much higher than in situ temperatures in the Arctic soils. However, the population in the Fibristel was better adapted to low temperatures, since at 0°C, it reached 30 % of its maximum activity, in contrast to MOB in the Historthel with only 5 to 9 %.