An emerging area of research investigates the ecological implications of repeatable individual differences in behavior. A personality is any behavior that is repeatable over time and across contexts. I examined inter- and intraspecific variation in personality in Peromyscus leucopus noveboracencis, the white-footed mouse, and P. maniculatus gracilis, the woodland deer mouse, as a mediator of coexistence and dispersal. I used open-field trials and principal component analysis to extract axes that describe activity, sociality, aggression, and location. I then used linear, generalized linear, and mixed effect models to reveal that P. maniculatus was more active and social than P. leucopus. In dyadic trials, sociality and aggression of the focal mouse were independent of the species of the opponent mouse. Analyses with raw variables indicated that both species approached heterospecifics more than conspecifics, and retreated from P. maniculatus more than from P. leucopus. Because extreme over-winter mortality left my study area almost vacant, I used behavioral axes to examine arrival date in dispersers. Early dispersers were more active, social, and submissive than late-season dispersers. Activity and sociality were also plastic over time, with the trend among all mice being to reduce activity and sociality as the season progressed. These studies illustrate that personality may be an axis of niche differentiation and is important in describing dispersal phenotypes in Peromyscus, thus illustrating the value of including personality in ecological studies.