The effects of a range of thermoperiods and soil temperatures upon growth of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) seedlings were studied. The seeds, of varieties glauca and menziesii, came from eight widely separated areas. Plants from both varieties made maximum growth with soil and air temperatures between 18 and 24 C during the 20 weeks of the study. Little requirement for diurnal fluctuation in temperature was evident. Low soil temperatures greatly reduced growth and hastened dormancy of plants grown under all the thermoperiods tested. The effects of a range of thermoperiods and soil temperatures upon the initiation of dormancy in var. menziesii and var. glauca seedlings grown under a 9-hour daily photoperiod also were investigated. Low air temperatures generally postponed initiation of dormancy of var. menziesii seedlings, but warm days and cool nights were most effective in postponing dormancy of var. glauca plants. Soil temperatures had no consistent effect upon the initiation of dormancy under the 9-hour photoperiod.