Measurements of CO2 exchange at the forest floor were made in shady dry areas dominated by feather mosses and open wet areas dominated by Sphagnum mosses within a 120-year-old black spruce forest in central Saskatchewan during 1996. Our objective was to study the environmental regulation of moss photosynthesis and forest floor (moss and soil) respiration, in order to calculate the contribution of moss photosynthesis to the carbon balance of the forest ecosystem. We used chamber gas exchange techniques to measure net carbon exchange at the forest floor during three periods that spanned the growing season. Measurements were also made with the clear chamber darkened, to obtain total respiration rates. Maximum measured rates of forest floor CO2 efflux were approximately 7 μmol m−2 s−1 in both feather moss and Sphagnum areas. We developed and applied a two-term (shallow component and deep component) exponential model to describe the temperature dependence of forest floor respiration. We interpret the shallow stratum component as live moss respiration, and our field estimates of moss respiration were consistent with laboratory studies on isolated moss samples. The average (±S.D.) proportion of total respiration originating from the shallow stratum (at a uniform soil temperature of 10°C) was 7.0±15% in the feather moss community and 21±20% in the Sphagnum community. Gross photosynthesis was calculated as the difference between chamber measurements made in the light and dark. A rectangular hyperbole was fitted to the light response curves for moss gross photosynthesis. Sphagnum had higher maximum rates of gross photosynthesis than feather moss, and Sphagnum showed a pronounced seasonal change in photosynthetic capacity. The photosynthesis and respiration models were used, along with environmental data, to calculate the carbon balance for the forest floor. Our data indicated that the forest floor lost 255.4 g C m−2 during May–October. This net loss of carbon was the difference between moss gross photosynthesis of −140.7 g C m−2 and total forest floor respiration of 396.1 g C m−2 during May–October. Moss photosynthesis contributed approximately 13% of the total ecosystem gross production. We calculated moss net primary productivity to be 228 and 80 g C m−2 in Sphagnum and feather moss communities, respectively.