We deployed CO2 and O2 sensors on the U.S. continental shelf off Cape Hatteras, North Carolina, during late summer 1994. A continuous 32-d gas record was obtained at 20 m in 25 m of water, below the thermocline for most of the period. Analysis of the correlation between CO2 and O2 indicates that biological and advective processes dominated the gas variability, with small or insignificant fluxes due to air-sea exchange, vertical eddy diffusion, and carbonate dissolution or formation. The observed O2: CO2 correlation was 1.39, within the range predicted for the photosynthetic quotient. Photosynthesis and respiration appeared to be tightly coupled, resulting in no net community production in these waters during the late summer. It is evident from these results that the combination of mooring-based CO2 and O2 measurements will be a powerful tool for studying the marine carbon cycle.