Abstract Peromyscus difficilis (J. A. Allen, 1891) is a cricetid rodent commonly called the southern rock deermouse or Zacatecan deermouse. It is of medium body size for the genus (28–43 g), with large ears and a long, evenly bicolored tail that is always slightly longer than the head plus body length. It is 1 of 56 species in the genus Peromyscus and includes 5 subspecies. P. difficilis is a Mexican endemic distributed throughout the Sierra Madre Occidental and Sierra Madre Oriental, southward through the mountainous regions of Guanajuato, Puebla, Hidalgo, and Veracruz, into northern and central Oaxaca. Habitat preferences range from dry, semiarid hills to montane forests. P. difficilis is not of special conservation concern.