The exchange of CO2between the atmosphere and tropical forest ecosystem of American Samoa was continuously monitored for 3 days in December 1988. The island was modeled as a simple wind‐ventilated respiration chamber with CO2input concentration, CO2output concentration, and residence time as measured variables. Net ecosystem production rates were calculated by incorporating Gaussian vertical diffusion into the model. Nighttime respiration averaged 0.34 (±0.06) g C m−2h−1; peak midday uptake was 0.85 (±0.23) g C m−2h−1. Thirty‐nine percent (±30%) more carbon was assimilated by the ecosystem during the day than was released at night. The diurnal net ecosystem production averaged 1.5 g C m−2d−1. The daytime CO2exchange rate varied as the logarithm of incident solar radiation over a range of 30 W/m2to 900 W/m2, with r2= 0.87. Total ecosystem respiration equaled photosynthesis at a radiation intensity of 72 (±14) W/m2. The saturation int