Density in the middle crown of the tree is a widely used index of abundance of the Douglas-fir tussock moth (Orgyia pseudotsugata [McDunnough]). Midcrown density is usually estimated by the destructive sampling of branches in the middle crown, but in sparse populations it can also be predicted by observing the frequency of occurrence of larvae or pupae on lower branches. Predictions are made from a prior calibration between the proportion of infested samples in lower branches and midcrown density. Two models for calibration were developed: an empirical relation based on regression analyses of population data, and a theoretical relation derived from the frequency distribution of individuals in a Poisson series and the vertical distribution of insects in the tree. The models produced almost identical results in predicting midcrown densities from estimated proportion values. The theoretical model is recommended because of its versatility for different-aged larvae and generality over a wider range of low densities. For. Sci. 33(1): 145-156.