Atmospheric CO2 exchange was measured in a boreal minerotrophic patterned fen in central Saskatchewan, Canada, using the eddy correlation technique. The study was conducted from mid-May to early October 1994, as part of the Boreal Ecosystem-Atmosphere Study (BOREAS). Herbaceous vegetation was dominated by buckbean (Menyanthes trifoliata) and various species of sedges (Carex and Eriphorum spp). Bog birch (Betula pumila) and willow species (Salix spp.) were dominant shrubs. Brown mosses were the predominant nonvascular vegetation. Canopy photosynthesis approached light saturation for PAR (photosynthetically active radiation) above 1000--1200 μmol m-2 s-1. High temperature (>20 ¿C) and vapor pressure deficit (>1.5 kPa) decreased photosynthesis significantly. On cool days with low vapor pressure deficit, canopy photosynthesis tended to follow incident PAR. The diurnal pattern of canopy photosynthesis exhibited a midmorning maximum on days with high temperature and vapor pressure deficit. Canopy photosynthesis reached a peak of 0.59 mg CO2 m-2 s-1 (midday) in early July, corresponding to the period of maximum leaf area index. Another increase in photosynthesis occurred in late August as the canopy recovered from a brief rise in water table that inundated some of the leaf area. The daily net CO2 exchange showed significant day-to-day variability resulting from changes in environmental conditions. The integrated value of the net ecosystem-CO2 exchange during the measurement period (mid-May to early October) was about 88 g C m-2. Consistent with the high productivity and high water table, this fen exhibited magnitudes of CO2 exchange larger than other northern wetlands reported in the literature. ¿ 1997 American Geophysical Union