Surface fluxes were measured over an old black spruce forest in the southern study area in 1994 between May 24 and June 4 and between July 19--29 and September 8--17 during three of the intensive field campaigns (IFCs) of the Boreal Ecosystem-Atmosphere Study (BOREAS). Eddy covariance measurements of latent and sensible heat, CO2, and momentum were carried out from a tower at 20 m above the ground. The evaporative fraction, defined as the ratio of latent heat flux over available energy, ranged from 41% (IFC 1) to 47% (IFC 2) and was relatively constant within each IFC (coefficients of variation <23%). During rapidly changing radiation conditions, the sensible heat flux varied with net radiation, while the latent heat flux was relatively constant. The mean evaporation at the old black spruce stand ranged from 1.44 mm d-1 during IFC 3 to 3.32 mm d-1 during IFC 2. Net CO2 fluxes were similar during IFC 1 and 2, with a maximum gain of 12.7 g CO2 m-2 d-1, but it was 20% lower during IFC 3. Increased respiration of the forest stand during IFC 2, which is associated with higher air and soil temperature, seems to account for the lack of increase in net CO2 influx between IFC 1 and 2. Nighttime CO2 fluxes measured by eddy covariance were often underestimated because of the light wind conditions. ¿ 1997 American Geophysical Union