Each of the two species of _Peromyscus_ illustrates one or more adaptations to its environment. _P. truei_ is adapted to climbing by possession of long toes, a long tail, and large hind feet. The tail is used as a counterbalance when climbing (Horner, 1954). When frightened, individuals of _P. truei_ often ran across the ground in a semi-saltatorial fashion, bounding over clumps of grass that were as much as 18 inches high. Such individuals usually ran to the nearest tree and climbed to branches 10 to 20 feet above the ground. Large eyes are characteristic of the _truei_ group of mice, and may be an adaptation to a semi-arboreal mode of life. A similar adaptation is shared by some other arboreal mammals, and of arboreal snakes. The large eyes of _P. truei_ in comparison to those of _P. maniculatus_, probably increase the field of vision, and permit the animal to look downward as well as in other directions. The above-mentioned adaptations of _P. truei_ permit these graceful mice to use their environment effectively. By climbing, this species can nest above-ground in the hollow branches of trees, and can rear its young in a comparatively safe setting. The ability to climb also permits vertical as well as horizontal use of a limited habitat. Because of the three-dimensional nature of the home range of _truei_, its range is actually larger than that of _maniculatus_ although the standard trapping procedures makes the home range of the two appear to be about the same size. Finally, trees may offer safety from predators, and a source of food that probably is the winter staple of this species. _Peromyscus maniculatus_ has adapted differently to its environment. Small size of body and appendages permit this species to use a variety of nesting sites and hiding places even though it is restricted, by its anatomy, to life on the ground. The tail and hind feet are shorter than in _P. truei_, and _P. maniculatus_ is an inefficient climber. I have placed individuals in bushes, and found that many walk off into space from a height of several feet. Perhaps the relative smallness of their eyes accounts for their seeming lack of awareness of how high they are above the ground. When frightened, individuals of _P. maniculatus_ ran rapidly in a zig-zag path and dove into the nearest cover. Mice, released from live traps, often stuck their heads under leaves, leaving their bodies exposed. This species tends to hide as rapidly as possible, and remain motionless. This tactic would not be of much value as an escape from carnivores, but it could be effective against birds of prey.