The effect of temperature and humidity on the toxicity of propoxur to adult female German cockroaches, Blattella germanica (L.), was examined. With high humidity, propoxur was found to be least toxic near 25°C. At either temperature extreme (10 or 35°C), increased mortality indicated a quadratic relationship between temperature and mortality. Mortality also increased at low humidities. In contrast, the effect of temperature on the toxicity of propoxur to western spruce bud worm, Choristoneura occidentalis Freeman, was a simple linear relationship which increased with increasing temperature. Exposure time was also related to changes in the toxicity of propoxur to the spruce budworm in both a linear and quadratic fashion. Multiple logit models were used to evaluate these multifactor toxicity experiments.