White-footed mice (Peromyscus leucopus) and deer mice (P. maniculatus) were studied under both allotopic and syntopic conditions in northeastern Kansas. These species typically occupy different habitats with the slightly larger P. leucopus inhabiting wooded areas, and the smaller P. maniculatus inhabiting grasslands. We recorded external measurements of each species in distinct habitats where only one species was typically present. Results were compared to measurements obtained from a grassland area recently invaded by woody plants where both species occur in syntopy. Under allotopic conditions, the two species were morphologically distinct and individuals could be readily assigned to one of the two species using a combination of measurements. Under syntopic conditions, however, many individual Peromyscus were intermediate in external measurements when compared to the allotopic populations, and could not easily be classified into one of the two species. Morphologically intermediate animals in the syntopic population could be the result of smaller than average adult P. leucopus occupying the area. The early successional habitat of the syntopic population is not typical for P. leucopus, but the recent invasion of woody plants may provide adequate habitat for smaller individuals, which may have been pushed out of the surrounding woodlands by larger individuals.