The Lena River is one of the biggest Russian rivers draining into the Laptev Sea. Due to predicted increasing temperatures the permafrost areas surrounding the Lena will melt at increasing rates. With this melting high amounts of carbon, either organic or as methane will reach the waters of the Lena and the adjacent Laptev Sea. As methane is an important green house gas its further fate in the Lena Delta is of uttermost importance. Methane oxidation by methanotrophic bacteria is the only biological way to reduce methane concentrations. However, the polar estuary of the Lena River is a challenging environment, with strong fluctuations in salinity and temperature. We determined the activity and abundance of aerobic methanotrophic bacteria (MOB), as well as their population structure. Activity was determined with 3H-CH4 as radioactive tracer, abundance was determined with quantitative PCR and the population structure was characterized by a fingerprinting method (MISA). Methane concentrations were rather low (41 ± 44 nM), as well as methane oxidation rates (1.1 ± 1.6 nM/d). In polar water (cold and saline) highest activities were found, whereas the highest abundance of MOB was in surface waters. The relation between methane turnover and abiotic factors will be used to characterize the eco-physiology of these polar and estuarine methanotrophs