The variability in the developmental rates for the embryos, larvae, and pupae of the western spruce bud worm, Choristoneura occidentalis, was skewed and was well described by a gamma probability density function. At extreme temperatures, the shapes of the frequency distributions for the embryos and pupae were not unimodal, suggesting the presence of thermal biotypes. Variability increased markedly towards the temperature extremes (15 and 31°C) and was greater for female than for male larvae. The shapes of the frequency distributions for larvae terminating diapause ranged from a skewed curve for second-instar larvae held in diapause at O'C to a negative exponential curve for larvae held at 5°C. A Monte Carlo simulation model showed that the probability of synchronous emergence of male and female moths was relatively constant (ca. 66%) over a range of average minimum/maximum temperatures. During the larval developmental period, average minimum/ maximum ranged from 2.3/18.1 to 6.0/21.O'C. For the pupae developmental period, average minimum/maximum temperatures ranged from 6.0/21.0 to 1l.4/27.1°C. Above these temperatures, the probability of synchronous emergence decreased.