Populations of the western spruce budworm, Choristoneura occidentalis, were sampled in western Montana, central Idaho, and eastern Oregon. In 22 of 24 populations, the proportion of females among pupae did not differ from 0.5. Similarly, in 21 of 24 populations, the proportion of females among adults was not different from 0.5. In all populations, differences between survival rates of male and female pupae were attributed to chance. The proportion of females among pupae (a) did not differ between insects on Douglas-fir and those on grand fir, (b) was slightly but significantly higher among insects in the mid-crown than among those in the lower crown, and (c) was significantly higher among the insects in the one site studied where average defoliation of current-year shoots was greater than 50%. Variation in the proportion of females among adults should have a negligible effect on the dynamics of these populations.