_Measurements._--Measurements of 3 males and mean and extreme measurements of 11 females, all from the type locality, are, respectively, as follows: total length, 192, 188 (incomplete), 196 (incomplete), 193 (188-209); length of tail, 102, 97 (broken), 97 (broken), 101 (94-114); length of hind foot, 22, 23, 23, 22.5 (22-23); length of ear, from notch, in flesh, 21, 22, 23, 21.5 (20-23); greatest length of skull, 27.4, 27.7, 27.9, 27.3 (26.5-28.3); basilar length, 20.2, 21.0, -- (broken), 20.4 (19.6-21.2); greatest breadth of braincase, 12.8, 12.8, 13.3, 12.85 (12.4-13.4); least interorbital breadth, 4.4, 4.6, 4.6, 4.41 (4.2-4.6); length of nasals, 10.1, 10.3, 10.9, 10.3 (9.8-11.1); diastema, 6.6, 7.0, 7.1, 6.78 (6.3-7.2); length of incisive foramina, 5.6, 5.9, 6.0, 5.77 (5.5-6.0); length of palatal bridge, 3.8, 3.9, --, 3.96 (3.8-4.3); postpalatal length, 9.9, --, --, 9.7 (9.2-10.4); alveolar length of maxillary tooth-row, 4.1, 4.1, 4.4, 4.2 (4.1-4.4). All measurements are in millimeters. _Measurements of the type._--Total length, 189; length of tail, 95; length of hind foot, 22; length of ear, from notch (in flesh), 21; greatest length of skull, 26.9; basilar length, 20.3; greatest breadth of braincase, 13.0; least interorbital breadth, 4.4; length of nasals, 10.1; diastema, 6.6; length of incisive foramina, 5.5; length of palatal bridge, 3.9; postpalatal length, 9.8; alveolar length of maxillary tooth-row, 4.2. _Comparisons._--From _Peromyscus truei gentilis_ (specimens from 5 mi. N Durango, Durango; 4 mi. W Durango, Durango; and 8 mi. N & 1 mi. W Sombrerete, Zacatecas), the subspecies of the surrounding region, _P. t. erasmus_ differs in markedly darker coloration, sides and face less brightly washed with orange buff, dark eye ring and spot at base of vibrissae more conspicuous, higher incidence and greater extent of buffy pectoral spot. External measurements do not differ significantly. No consistent cranial differences were found. From _Peromyscus truei gratus_ (specimens from Pedregal de los Reyes, Distrito Federal, Mexico) to the southeast, _P. t. erasmus_ differs in slightly darker dorsal color, more inflated bullae, and less sinuous (not bulging so much forward laterally) anterior margin of interparietal. From _Peromyscus truei gratus_ (specimens from various localities in eastern Jalisco and western Michoacan) to the south, _P. t. erasmus_ differs in slightly darker dorsal color, longer ears, and more inflated bullae. From _Peromyscus truei truei_ (specimens from 4 mi. N El Rito, Rio Arriba Co., New Mexico) to the northwest, _P. t. erasmus_ differs in much darker color, shorter tail, shorter hairs on tail, smaller ears, shorter rostrum, wider nasals, and more pointed anterior ends of incisive foramina.