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Large body size in Keen’s mouse, Peromyscus keeni, has been regarded as a relictual character that developed in times of geographic separation from P. maniculatus. However, body-size changes in Keen’s mouse have not been studied in detail. To address this problem the present paper compares the size of ancient and modern Peromyscus specimens from Vancouver Island. Results indicate that Late Pleistocene Peromyscus from Arch-2 Cave and early Holocene Peromyscus from Pellucidar Cave are significantly larger than those of modern P. maniculatus and P. keeni. Morphology and linear discriminant analyses support tentative assignment of several ancient specimens to P. keeni. Radiocarbon age estimates of 11,960±45 BP (14,004–13,637 cal BP) on a small mammal bone and 12,370±35 BP (14,695–14,148 cal BP) on Ursus arctos from Arch-2 Cave place these faunas on the island as relative sea level fell from a postglacial highstand, suggesting a local source for faunas with limited over-water dispersal capacities. Results of this study are consistent with insular relictual gigantism in Keen’s mouse, although some modification of the original hypothesis is needed to explain the smaller size of modern than ancient mice.