Eddy covariance measurements of the carbon dioxide flux from an ombrotrophic bog near Ottawa, Canada, were made between June 1, 1998, and May 31, 1999. Net ecosystem exchange of CO2 (NEE) showed a distinct annual cycle, with net daily uptake increasing rapidly after snowmelt, peaking in midsummer and declining toward the fall. Summer (June to September) mean daily NEE flux was an uptake of -2.8¿0.23 (standard error) g CO2 m-2 d-1, but daily values ranged considerably from a loss of 4.8 g CO2 m-2 d-1 to a maximum uptake of -8.3 g CO2 m-2 d-1. Daytime fluxes of CO2 were closely related to the photosynthetically active radiation flux, with derived relationships varying monthly. A curvilinear relationship developed between nighttime NEE and soil temperature produced a Q10 value of 3.0. Throughout the late fall and the snow-covered periods (November 5 to April 6), mean daily fluxes showed a fairly constant efflux of ~1.1¿0.003 g CO2 m-2 d-1. The integrated non-growing season CO2 loss was 183 g CO2 m-2. However, this was offset by gains during the long growing period resulting in an integrated annual NEE net uptake of 248¿68 g CO2 m-2 yr-1 for this peatland. The growing season measurements of CO2 flux at this site were similar to those reported in the few previous studies on northern peatland ecosystems, but no previous annual estimates of NEE based on year-round measurements have been published for other peatlands. Compared to annual measurements at forest sites in North America, the net exchange at this site falls between that of a small annual loss recorded at a northern boreal spruce forest and the substantial uptakes measured at a temperate mixed forest and boreal aspen forest sites. ¿ 2001 American Geophysical Union