The annual balance of CO2 was measured on a subarctic flark fen located in northern Finland (69¿08'N, 27¿17'E). Measurements were conducted using the micrometeorological eddy covariance method during the period from April 1998 to April 1999. Special emphasis was put on the importance of the wintertime CO2 balance as a component of the annual budget. The CO2 fluxes observed during the winter were relatively small (0.0055 mg CO2 m-2 s-1), but due to the long duration of the snow cover period, they play an important role in the annual balance of CO2. The efflux during the coldest periods, when soil surface temperatures fell below -10 ¿C, was enabled by markedly higher peat temperatures (close to 0 ¿C) in deeper layers. The typical nighttime respiration observed in summer was 0.10 mg CO2 m-2 s-1. The net photosynthesis had a typical daytime peak value of about -0.20 mg CO2 m-2 s-1 in July. Corresponding with the instantaneous CO2 fluxes, the highest daily sink balances of about -9 g CO2 m-2 d-1 were observed in July, while the highest daily respiration balances of about 2.5 g CO2 m-2 d-1 were observed before and after the sink period. The CO2 balance for the winter period (105 g CO2 m-2) was greater than the absolute value of the total annual balance (-68 g CO2 m-2), which shows the importance of the wintertime efflux. After taking into account estimates for CH4 efflux and carbon leaching, an annual carbon balance of -7 ¿ 5 g C m-2 was obtained for the fen.