The Anastasia Island beach mouse and the Southeastern beach mouse are two of six existing coastal subspecies of the oldfield mouse (Peromyscus polionotus). The oldfield mouse is a wide-ranging species in the Southeast. One of the largest species of beach mice (averaging 138.5 millimeters in length and 53 millimeters in tail length), the Anastasia Island beach mouse is much paler than most inland races of the oldfield mouse.