Field observations and a laboratory test indicate that Peromyscus gossypinus is a more active climber than P. floridanus. Subjects conceived and raised in the laboratory tended to perform more poorly on the climbing test than did wild-caught animals. The greater arboreal tendency of gossypinus may contribute to its wider distribution and broader habitat tolerance, as compared to floridanus, by allowing it to exploit more effectively the resources of the environment. Difference in three-dimensional space utilization of the two species also may serve to reduce competition between them in areas of sympatry. The interspecific variation in climbing activity appears to involve both psychological and morphological adaptations, with the former being of relatively greater importance. Tail length is not correlated with climbing ability in these species. The principal morphological adaptation for climbing appears to be relative development of the plantar tubercles. Two isolated populations of floridanus from different habitat types showed greater divergence in climbing activity than gossypinus stocks from the same localities and habitats. The difference in climbing tendency of the two floridanus populations is believed to be related to the accessibility and general abundance of acorns in their respective habitats.