Net ecosystem CO2 exchange was measured during the 1990 growing season (June to August) along a transect starting 10 km inland from James Bay and extending 100 km interior to Kinosheo Lake, Ontario. Sites were chosen in three distinct areas: a coastal fen, an interior fen, and a bog. For the most productive sites in the bog, net daily uptake rates reached a maximum of 2.5 g C-CO2 m-2 d-1 with an area-weighted exchange of 0.3 g C-CO2 m-2 d-1 near midsummer. This site was estimated to be a net carbon source of 9 g C-CO2 m-2 to the atmosphere over a 153-day growing season. The interior fen was less productive on a daily basis with a net maximum uptake of 0.5 g C-CO2 m-2 d-1 and with corresponding area-weighted uptake of 0.1 g C-CO2 m-2 d-1 during midsummer. Early and late season release of carbon to the atmosphere resulted in a net loss of 21 g C-CO2 m-2 over the growing season from this site. The coastal fen was the most productive site with uptake rates peaking near 1.7 g C-CO2 m-2 d-1 which corresponded to an area-weighted uptake of 0.8 g C-CO2 m-2 d-1 during midsummer and an estimated net uptake of 6 g C-CO2 m-2 for the growing season. Associated with net CO2 exchange measurements, multispectral reflectance properties of the sites were measured over the growing season using portable radiometers. These properties were related to exchange rates with the goal of examining the potential for satellite remote sensing to monitor biosphere/atmosphere CO2 exchange in this biome. The normalized difference vegetation index (NDVI) computed from surface reflectance was correlated with net CO2 exchange for all sites with the exception of areas with large proportions of Sphagnum moss cover. These mosses have greater near-infrared reflectance than typical surrounding vegetation and may require special adjustment for regional exchange/remote sensing applications.