Climatological measurements, including carbon dioxide flux density, were made from April to September in 1994 and from April to November in 1996 at a fen wetland near Thompson, Manitoba, Canada, as part of the Boreal Ecosystem-Atmosphere Study (BOREAS). For both years, the study period was warmer and drier than the 24-year climate normals. The period of CO2 uptake was similar for both years, reaching maximum measured assimilation rates of -0.55 mg m-2 s-1 in midsummer. However, warmer air temperatures and an earlier snowmelt in the spring of 1994, which led to an earlier thaw for the fen surface, and warmer and drier conditions in the fall of 1994 promoted CO2 production at times when the vascular vegetation was not photosynthesizing. As a result, in 1994 over the study period of 124 days the fen was a net source of CO2-carbon to the atmosphere, losing 30.8 g C m-2; for the same period in 1996 the fen was a net sink of CO2-carbon, assimilating -91.6 g C m-2. Given the immense store of carbon in boreal peatlands and given a growing understanding of the relative importance of the soil carbon pool to net ecosystem exchange and of the sensitivity of this carbon storage to temperature and wetness, this boreal fen's response to earlier spring warming and drier conditions extends our understanding of the impact of climate change on the carbon balance for northern ecosystems. ¿ 1999 American Geophysical Union