Deer mice are definitely nocturnal. Of thousands of specimens trapped only a few were taken in the daytime. Two of these were taken in a rock slide at Pass Creek Pass, Pend Oreille County, on the same day, and some unusual conditions may have incited them to travel in daylight. Deer mice are active throughout the winter and their lacy footprints are seen on the snow in below-freezing weather. They are mainly terrestial but one race (_oreas_) is at least partly arboreal for it is often taken in traps set in trees for flying squirrels. One was caught in a trap set 50 feet from the ground. All are good climbers, and _oreas_ is perhaps the best. One was watched as it climbed the rough cedar shake wall of a cabin, ran upside-down across three feet of rough ceiling, and descended the opposite wall head first. They are equally agile in climbing rocks and cliffs. The ordinary gait of the deer mouse is a steady run. When frightened it may leap a distance of one foot or more. When surprised in its nest it usually remains in the vicinity, hopping about in confusion, returning time after time to the exposed nest. The food of deer mice consists principally of vegetable material such as seeds, nuts, berries, and the soft parts of fleshy plants. Mushrooms and lichens are eaten. Insect remains are present in small quantities in most stomachs examined. Flesh is also eaten and small mammals caught in traps are often eaten by deer mice. Cannibalism appears to be common only in captivity. [Illustration: FIG. 107. Distribution of the deer mouse in Washington. A. _Peromyscus maniculatus oreas._ B. _Peromyscus maniculatus austerus._ C. _Peromyscus maniculatus hollisteri._ D. _Peromyscus maniculatus gambelii._ E. _Peromyscus maniculatus artemisiae._] Deer mice are often heavily parasitized by tapeworms and roundworms. Virtually all of the deer mice taken on Jones Island in the San Juan Islands had livers so infected by the eggs of a nematode worm as to be swollen to several times their natural size, and had a yellow, crystalline appearance. The maggots of parasitic flies often occur in the anal and genital region of deer mice, effectively sterilizing some individuals. Predatory snakes, birds, and mammals doubtless kill and eat deer mice. Strangely enough, although deer mice may be the commonest mammal in an area, the pellets of owls collected in the same area usually contain the remains of relatively few deer mice. The impact of owls seems to be greatest upon mice, such as _Microtus_, which follow definite runways. The breeding season of the deer mouse extends over most of the spring and summer. Growth and development of the young of _Peromyscus m. oreas_ have been discussed by A. Svihla (1936A). Embryos found varied in number from 2 to 8 with a mean of 4.5. =Peromyscus maniculatus oreas= Bangs _Peromyscus oreas_ Bangs, Proc. Biol. Soc. Washington, 12:84, March 24, 1898.