In situmeasurements of CO2assimilation byDryas integrifoliaat different stages of development and under different environmental conditions were made on Devon Island, Northwest Territories.Dryascan fix CO2in excess of respiration over a 24-h period under conditions of clear nights and cloudy days. The maximum net assimilation rate measured was 4.2 mg g−1dry weight h−1. The maximum amount of CO2fixed in 24 h was 61.54 mg g−1dry weight. Maximum net assimilation occurred at 8 to 10 °C leaf temperatures. Positive net assimilation occurred at 1 °C leaf temperature. Light compensation was shown to be less than 0.04 langley min−1. Leaf temperatures were always greater than ambient. The maximum leaf temperature measured was 39 °C. Net assimilation rates appear to decrease as the season progresses.