_P. maniculatus_ lives almost everywhere in Mesa Verde; the preferred habitats are open and grassy with an overstory of mixed shrubs. Individuals of _P. maniculatus_ venture into ecotonal areas lying between grasslands and pinyon-juniper forest, or between sagebrush and pinyon-juniper forest. _P. maniculatus_ is found also in disturbed areas and in stands of sagebrush that occur in clearings of the pinyon-juniper woodland. In such areas, _P. maniculatus_ and _P. truei_ are sympatric; their home ranges overlap and any inter-specific competition that might occur would be expected in these places. The ability of _P. maniculatus_ to live in many different habitats is correlated in part with its ability to build nests in a variety of sites. Whereas _P. truei_ usually builds nests only in dead branches or logs, _P. maniculatus_ builds nests in such varied places as spaces under rocks, at the bases of rotten trees, and in abandoned tunnels of pocket gophers. This adaptability is advantageous for the dispersal of young individuals and the movement of adults into new areas. Nesting sites have important bearing on survival of the young. In Mesa Verde the rainy season occurs in July and August, while both species of _Peromyscus_ are reproducing. It is reasonable to assume that young animals that remain dry survive better than those that become wet and chilled. The nestling young of _P. truei_ are in a more favorable position to remain dry and warm than are nestling young of _P. maniculatus_. Captives of each species differed in the amounts of water consumed per gram of body weight. Individuals of _P. truei_ consumed more water per gram of body weight than individuals of _P. maniculatus_. Animals may drink more water than they require when allowed to drink _ad libitum_, but Lindeborg (1952) has shown that species which consume less water when it is not restricted also fare better on a reduced ration. _P. maniculatus_ appears to be better adapted to aridity than _P. truei_. The preferred habitats of each species are in accord with these findings. Within the trapping grid, the most moderate microenvironment, in terms of temperature and humidity, was in the pinyon-juniper forest, where _P. truei_ lives. The temperature extremes were wider in the microenvironments of a thicket of oak brush and of two different stands of sagebrush, where _P. maniculatus_ lives, than in the forest. _P. maniculatus_ tends to live in the harsher, more arid parts of Mesa Verde. Because of its propensity to build nests under things, or in the ground, and because of its ability to use less water per gram of body weight, _P. maniculatus_ is better adapted to withstand harsh environments than is _P. truei_.