We used an automated, multiplexing gas-exchange system to measure the net exchange of CO2 at the surfaces of three shady feather moss and three exposed sphagnum moss sites in a black spruce forest during 35 days at the end of the 1995 growing season. Midday gross photosynthesis was 0.5 to 1.0 μmol m−2 s−1 by feather moss and 0.5 to 2.5 μmol m−2 s−1 by sphagnum moss. Photosynthesis by sphagnum moss was reduced by approximately 70% at 0 °C, and reached a maximum rate at 8 °C. Nighttime CO2 efflux, the sum of soil and moss respiration was 1 to 2.5 μmol m−2 s−1 above feather moss and 0.5 to 1.5 μmol m−2 s−1 above sphagnum moss at moss temperatures of 0 to 15 °C. The higher rates of respiration at the feather moss sites probably reflected a greater belowground input of carbon from black spruce, and the lower rates of photosynthesis were probably associated with shading by the black spruce canopy. Photosynthesis by moss accounted for 10 to 50% of whole-forest gross CO2 uptake measured simultaneously by eddy covariance. Respiration at the moss surface was 50 to 90% of whole-forest respiration, with a decreasing fraction on warm nights apparently because of a disproportionate rise in aboveground respiration.