From 1959 through 1968, lungs from 124 Rocky Mountain bighorn sheep (Ovis c. canadensis) from the Sun River herd in western Montana were examined foT lungworm infections. All lungs were infected with Protostrongylus stilesi and 104 (84%) contained concurrent infections of P. stilesi and P. rushi. Significant correlations were observed between levels of lungworm infection and total rainfall during April, May, and June of each year. An explanation of this in terms of terrestrial snail (intermediate host) populations and a suggestion for the possible use of these data in developing a predictive model for forecasting lungworm levels for use in bighorn sheep management are given.