Development of the 1st-instar (L1) of the spruce budworm, Choristoneura fumiferana (Clemens), was studied at different temperatures under laboratory conditions. The development process was divided into different phases based on 4 distinct events observed during L1 development: hatching of L1, construction of hibernacula, excretion of green pellets, and molt. The green substance in newly hatched L1 was found to be proteinaceous. The proportion of L1 that successfully completed development at various temperatures indicated that the optimal temperature range for L1 development was between 18 and 27°C. Larval mortality, particularly at low temperatures, occurred mainly before construction of hibernacula and excretion of green pellets. Low temperatures had a weak effect on development time from larval hatching to construction of hibernacula, but the duration of subsequent development periods were substantially increased under these temperatures. Temperature threshold (TT ≈ 6°C), degree-day requirement (DD ≈ 87), and other developmental parameters were estimated.