Populustrichocarpa Torr. & Gray from Alaska was crossed with the same species from a Montana population, and the resulting intraspecific hybrid was crossed with Populusdeltoides Bartr. ex Marsh, from Minnesota. Ramets from the five parents and hybrids were tested for the response of leaf conductance to temperature of the prior night. Leaf stomatal density and size were determined and net photosynthesis was measured between 15 and 40 °C for all clones. Alaska and Montana sources of P. trichocarpa and their hybrid showed mildly sensitive leaf conductance responses to 10 °C nights and comparable responses to 4 °C nights. The Alaska parent had an optimum rate of net photosynthesis at 20 °C, 10 °C lower than that of its hybrid, but the Montana parent's temperature response curve was less clearly defined. Although the P. trichocarpa hybrid was intermediate or similar to both parents in temperature adaptation, in stomatal density, size, and Q-ratio it was more similar to the Alaska parent. The P. deltoides parent and its interspecific hybrid exhibited a sensitive pattern of leaf conductance response to cool nights. The hybrid had a net photosynthetic temperature response curve that closely paralleled that of the P. deltoides parent, yet rates were lower, between 20 and 35 °C, in both compared with the P. trichocarpa parent. The interspecific hybrid more closely resembled the P. deltoides than the P. trichocarpa parent in overall temperature adaptation and had morning leaf conductance higher than that of both. Stomatal density and size were more unlike the P. deltoides than the P. trichocarpa parent, and the Q-ratio of the hybrid was intermediate to that of the parents.