Of two species, the one producing the more young probably would be subjected to more parasitism and predation than the species producing fewer young. A favorable season for botflies, _Cuterebra_ sp., revealed that _P. maniculatus_ has a higher incidence of parasitism by these flies than has _P. truei_; possibly the adult flies concentrate in the open, grassy areas where _P. maniculatus_ is more abundant, rather than in the woodlands where _P. truei_ lives. Perhaps the lower parasitism of _P. truei_ by warbles is related to the physiology of this species of mouse. Near Boulder, Colorado, the incidence of infection by warbles is lower in _P. difficilis_, a species closely related to _P. truei_, than in _P. maniculatus_ (V. Keen, personal communication). Although predation by carnivores would be expected to be higher on _P. maniculatus_, because this species does not climb, my data show that more individuals of _P. truei_ were taken by coyotes. I lack confidence in these findings, suspecting that another sample might indicate the reverse. Birds of prey probably catch more individuals of _P. maniculatus_, because this species lives in more open habitats. My data do not warrant firm conclusions regarding predation. The length of time females must care for their young influences the rate at which individuals can be added to the population. Females of _P. truei_ nurse their young longer and keep them in the nest longer than do females of _P. maniculatus_. Although this may enhance the chances of survival of young of _P. truei_, it also reduces the number of litters that each female can have in each breeding season. Females of _P. maniculatus_ can produce more young per litter, and each female probably can produce more litters per year than females of _P. truei_. Captives of _P. truei_ were tolerant of other individuals of the same species, even when kept in close confinement. However, when there was slight shortage of food or water they killed their litter mates, or females killed their young. Only a short period of time was necessary for one mouse to dispatch all others in the litter. The attacked mice were bitten through the head before being eaten; the brains and viscera were the first parts consumed. The population might be decimated rapidly if drought forced this species to cannibalism. When the supply of food or water was restored, the captive mice resumed their tolerant nature. In captivity, _P. maniculatus_ is amazingly tolerant of close confinement with members of the same species; individuals did not tend to kill their litter mates, or their young, even during shortage of food and water. This tolerance, especially under stressful conditions, probably enables _P. maniculatus_ to persist in relatively unfavorable areas. Adaptations to Environment