My primary purpose in undertaking a study of the two species of _Peromyscus_ was to analyze a number of ecological factors influencing each species--their habitat preferences, how the mice lived within their habitats, what they ate, where they nested, what preyed on them, and how one species influenced the distribution of the other. In general, my interest was in how the lives of the two species impinge upon each other in Mesa Verde. Physiography The Mesa Verde consists of about 200 square miles of plateau country in southwestern Colorado, just northeast of Four Corners, where Colorado, New Mexico, Arizona and Utah meet. In 1906, more than 51,000 acres of the Mesa Verde were set aside, as Mesa Verde National Park, in order to protect the cliff dwellings for which the area is famous. The Mesa Verde land mass is composed of cross-bedded sandstone strata laid down by Upper Cretaceous seas. These strata are known locally as the Mesaverde group, and are composed, from top to bottom, of Cliff House sandstone, the Menefee formation, the Point Lookout sandstone, the well known Mancos shale, and the Dakota sandstone, the lowest member of the Cretaceous strata. The Menefee formation is 340 to 800 feet thick, and contains carbonaceous shale and beds of coal. There are surface deposits of Pleistocene and Recent age, with gravel and boulders of alluvial origin; colluvium composed of heterogeneous rock detritus such as talus and landslide material; and alluvium composed of soil, sand, and gravel. A layer of loess overlays the bedrock of the flat mesa tops in the Four Corners area. The earliest preserved loess is probably pre-Wisconsin, possibly Sangamon in age (Arrhenius and Bonatti, 1965:99). The North Rim of Mesa Verde rises majestically, 1,500 feet above the surrounding Montezuma Valley. Elevations in the park range from 8,500 feet at Park Point to about 6,500 feet at the southern ends of the mesas. The Mesa Verde land mass is the remnant of a plateau that erosion has dissected into a series of long, narrow mesas, joined at their northern ends, but otherwise separated by deep canyons. The bottoms of these canyons are from 600 to 900 feet below the tops of the mesas. The entire Mesa Verde land mass tilts southward; Park Headquarters, in the middle of Chapin Mesa (Fig. 1), is at about the same elevation as is the entrance of the park, 20 miles by road to the north.