Concentrations of atmospheric CO2were measured continuously throughout the 1966 thaw season at two stations on the Arctic coastal plain of Alaska. A seasonal decrease in CO2of 15 ppm was demonstrated, as well as changes in concentration associated with storm frequencies and diurnal gradients. Correlation between rate of plant growth and declining CO2concentration in the atmosphere was high (r = .98). Tundra growth was initiated by mid—June and was 90% completed in mid—July. Standing crop above ground and below ground was estimated to be 82 and 100 g/m2, respectively. A 23% turnover of 273 g/m2of litter was confirmed. Gross primary production was estimated at 344 g/m2. An annual budget for CO2in an Arctic biosphere is postulated and efficiencies for both energy and carbon storage calculated. Based on seasonal net radiation a photosynthetic efficiency of 0.7% was determined, whereas atmospheric CO2was apparently used at the rate of nearly 12%. Estimates of CO2turnover due to biological activity are similar to the 10—year residence time of a molecule before absorption in the sea.