Mr. Fred E. Mang Jr., Photographer, National Park Service, processed large numbers of photomicrographs of plant epidermis. Dr. Kenneth B. Armitage, The University of Kansas, offered valuable suggestions for the study of water consumption in the two species of _Peromyscus_, and permitted me to use facilities of the Zoological Research Laboratories at The University of Kansas. Dr. Richard F. Johnston, The University of Kansas, permitted me to house mice in his controlled-temperature room at the Zoological Research Laboratories. I am grateful to all of the above mentioned persons for their aid. I acknowledge with gratitude the guidance, encouragement, and critical assistance of Professor E. Raymond Hall throughout the course of the study and preparation of the manuscript. I also extend my sincere thanks to Professors Henry S. Fitch, Robert W. Baxter, and William A. Clemens for their helpful suggestions and assistance. To my wife, Virginia, I am grateful for encouragement and assistance with many time-consuming tasks connected with field work and preparation of the manuscript. Travel funds provided by the Kansas Academy of Science permitted me to work in the park in August, 1965. The Wetherill Mesa Project was an interdisciplinary program of the National Park Service to which the National Geographic Society contributed generously. I am indebted to the Society for a major share of the support that resulted in this report. This is contribution No. 44 of the Wetherill Mesa Project. DESCRIPTIONS OF MAJOR TRAPPING LOCALITIES Trapping was begun in September of 1961 in order to analyze the composition of rodent populations within the park. I used the method of trapping employed by Calhoun (1948) in making the Census of North American Small Mammals (N. A. C. S. M.). It consisted of two lines of traps, each 1,000 feet long having 20 trapping stations that were 50 feet apart. The lines were either parallel at a distance of 400 feet from each other, or were joined to form a line 2,000 feet long. Three snap traps were placed within a five-foot radius of each station, and were set for three consecutive nights. More than a dozen areas were selected for extensive trapping (Fig. 1). Some of these were retrapped in consecutive years in order to measure changes in populations. One circular trapline of 159.5 feet radius was established in November 1961, and was tended for 30 consecutive days to observe the effect of removing the more dominant species (Calhoun, 1959). Other mouse traps and rat traps were set in suitable places on talus slopes, rocky cliffs, and in cliff dwellings. Most of these traps were operated for three consecutive nights.