Net ecosystem exchange (NEE) of carbon dioxide (CO2) was measured at a temperate cattail marsh using the eddy covariance technique in order to examine the relationships between NEE, weather, and vegetation properties. Analyses of CO2 fluxes for a complete year (May 9, 2005 to May 30, 2006) showed that the marsh wetland was a net CO2 sink for each month from June to September (monthly 24-h averages of −0.1, −5.1, −4.8, and −2.2gCm−2 day−1, respectively) and a source of CO2 to the atmosphere for the remaining fall and winter months. Wintertime (November to April) average NEE was 0.5gCm−2 day−1. The annual cumulative CO2 balance shows a net uptake of 264gCm−2 year−1 by the marsh. Ecosystem respiration (ER) and gross ecosystem production (GEP) were estimated to be 567 and 831gCm−2 year−1, respectively. Respiration rates were associated with a Q 10 value of 2.8. Peak aboveground biomass (average of 1156gm−2) and peak green LAI (average of 3.63) were reached in mid-August. Variations in growing season NEE were well correlated with variations in live biomass (r =0.92) and green LAI (r =0.94).