The first continuous multi-year measurements of the CO2 exchange between a subarctic fen and the atmosphere were conducted at Kaamanen in northern Finland (69°N). According to our six-year data-set, the fen is presently a sink of atmospheric CO2 with a mean rate of −22 g C m−2 yr−1. The interannual variation of the CO2 balances originates almost completely from the variations during the snow-free period, but the efflux in the wintertime constitutes a significant part of the annual balance. The snow melt timing is the most important single determinant of the annual carbon balance. In contrast to a commonly-held view, the hydrometeorological conditions during the growing season had only a minor effect on the annual balance, emphasizing the importance of year-round measurements. This study indicates that climate warming may increase the length of the growing season and thus benefit rather than threaten the carbon pool of subarctic peatlands.