_Peromyscus pectoralis_ and _P. boylii_ are closely related morphologically and seem to occupy the same habitat. In the Sierra Madre Oriental, according to the field notes of the collector (Heinrich, June 6 to August 5, 1953), individuals of _P. pectoralis_ had a pinkish coloration on the mouth and forefeet produced by the juice of the "nopal" cactus fruit, on which obviously the mice feed, whereas only a few specimens of _boylii_ were thus discolored. It was noted that _boylii_ was feeding on acorns. Furthermore, the two species may differ in time of breeding; in August, males of _pectoralis_ had the testes well developed when those organs were small in _boylii_ collected at the same locality. A specimen from 53 kilometers north of El Limón, was shot at a height of 10 feet on a concrete underpass. Other specimens were taken in a trap line that yielded _Peromyscus boylii_, _P. leucopus_ and _Liomys irroratus_. Two subspecies of _P. pectoralis_ occur in Tamaulipas: _P. p. collinus_ is widely distributed in the central and western parts of the state and _P. p. eremicoides_ occurs only in the western "corner" of the state. =Peromyscus pectoralis collinus= Hooper 1952. _Peromyscus pectoralis collinus_ Hooper, Jour. Mamm., 33:372, August 19, type from San José, 2000 ft., Sierra San Carlos, 12 mi. NW San Carlos, Tamaulipas. _Distribution in Tamaulipas._--Along the central and western mountains. A female obtained on January 21 at a place 53 kilometers north of El Limón, contained three embryos. A lactating female was taken on August 2 in the Sierra Madre Oriental. Males, as previously noted, had well-developed testes in August. The weights of 17 males and 20 females from the Sierra de Tamaulipas were, respectively, 26.6 (24-33), and 25.6 (21-31) grams. Measurements of specimens from different localities in Tamaulipas averaged about the same, except that those of specimens from Palmillas, averaged smaller. The small size suggests intergradation between the subspecies _collinus_ and _eremicoides_. The latter occurs to the west and differs from _collinus_ in smaller size, more grayish coloration, completely white tarsal joint and relatively longer tail. Hooper (1952:374) reported specimens from Jaumave as intergrades between the two subspecies before mentioned and Osgood (1909:164) identified two specimens from there as _eremicoides_. In the present account, individuals from Palmillas and Jaumave are referred to _collinus_.