Five species of _Peromyscus_ inhabit Mesa Verde National Park (Anderson, 1961). Two of these species, _P. crinitus_ and _P. difficilis_ are rare, and none was taken in more than 14,000 trap nights. Several individuals of _P. boylei_ were taken in live traps, but this species could not be regarded as common. The two remaining species, _P. truei_ and _P. maniculatus_, are the most abundant species in the park. Comparison of the habitats and life-cycles of these two forms and analyses of their interrelationships have been the objectives of this study. The distribution of _P. truei_ in the park is regulated by the presence of living pinyon-juniper woodland where logs and hollow trees of _Juniperus osteosperma_ provide nesting and hiding places, and where seeds of juniper trees and nuts of pinyon trees provide food. Several other investigators have reported _P. truei_ to be associated with trees, but apparently these findings have not assumed the importance they warrant in understanding the ecology of this species. Bailey (1931:152) observed an individual of _P. truei_ nesting in a tree on Conchas Creek, New Mexico, and thought that this species might be more arboreal than was generally supposed. The type specimen of _P. t. truei_ was taken by Shufeldt from a "nest protruding from an opening in the dead and hollow trunk of a small pinon, at least 2 feet above the ground.... The nest, composed of the fine fibers of the inner bark of the pinon, was soon pulled out, and its owner dislodged...." (Shufeldt, 1885:403). Individuals of _P. truei_ usually build nests in trees, or in hollow logs, and are therefore more abundant in pinyon-juniper woodland where there are many such nesting sites. Rocks and stones are not necessary in the habitat of _P. truei_, although this species was most abundant where there was stony soil. The coincidence of rock or stones and a high density of _P. truei_ is thought to be explainable in terms of vegetation. Stony soils support mixed shrubs as well as pinyon and juniper trees; the additional cover and source of food probably allow a greater abundance of _P. truei_ than would be possible without the shrubs. Secondarily, the rock provides nesting sites for more mice. Stands of mixed shrubs, lacking a pinyon-juniper canopy, do not support _P. truei_. Its absence was noteworthy on Navajo Hill and on the northern end of Wetherill Mesa where only _P. maniculatus_ lived among the mixed shrubs and grassland. On the Mesa Verde, pinyon and juniper trees must be present in order for _P. truei_ to live in an area; and, these trees must be alive. Dead pinyons and junipers still stand in the burned part of Morfield Ridge, but no _P. truei_ were found there. Although a few individuals of _P. truei_ were taken in stands of sagebrush adjacent to pinyon-juniper woodlands, this species does not ordinarily venture far from the forest.