Territoriality and its function in mountain lions was studied in the Idaho Primitive Area in central Idaho. This paper details the preliminary findings of the first four years of this study. Research indicated that the minimum size of male territories remained constant from year to year, while female ranges varied depending on their reproductive status. Males used larger areas, and resident males occupied distinct territories without overlap. Resident females on the other hand shared some common areas. Territorial defense was not observed transient lions of both sexes moved freely through occupied territories. Instead a mutual avoidance behavioral mechanism, stimulated by both visual and olfactory marks, served to distribute lions in both time and space, and thus prevent any territorial conflicts.