Developmental rates of embryos, larvae, and pupae of the western spruce bud worm, Choristoneura occidentalis, were determined over temperatures ranging from 10 to 31'C and RHs ranging from 10 to 100%. Humidity had a trivial influence on developmental rates of all these life stages, whereas temperature had major effects. Several nonlinear developmental rate models and a cubic polynomial model were fit to the data, all of which adequately described the data for each life stage. Development was minimal below IO'C for all stages, and maximum rates occurred from 26 to 29'C, depending upon sex and life stage. At temperatures above 29'C, the rates decreased sharply. Low temperatures induced melanin production in over 30% of the larvae and pupae. Survival of embryos and pupae was determined over temperatures and humidities as noted above. Embryonic survivorship decreased at the temperature extremes and as the humidity decreased. Survival was highest near 220C and 100% RH. Pupal survivorship decreased at both temperature and humidity extremes. Rates were highest near 23'C and 75% RH. Survivorship of diapausing larvae decreased at the humidity extremes (10 and 100% RH) and decreased with the length of time in diapause (7 versus 9 months).