_Diagnosis._--Size medium for the genus (see measurements); tail about as long as head and body; dorsal coloration near Sayal Brown (capitalized color terms after Ridgway, 1912); sides reddish; underparts Light Buff; tail bicolored but not distinctly so; supraorbital border of skull angular, and bullae pointed anteriorly; anterior half of braincase nearly straight (not rounded) as viewed from above; upper molar series long (4.7-5.0); incisive foramina short in relation to length of skull. _Comparisons._--From _Peromyscus boylii_, _P. aztecus_ differs as follows: Larger in most parts measured; maxillary tooth-row 4.7-5.0 instead of 4.0-4.6; color brighter on sides (reddish instead of ochraceous); supraorbital border angular instead of rounded; anterior border of zygomatic plate convex in upper half and almost straight in lower half as opposed to nearly straight throughout in _boylii_; pterygoid fossa broader; bullae more pointed anteriorly and less inflated; mesostyles of upper molars larger; surface between orbital region and nasals convex in lateral view instead of flat. _Remarks._--When Saussure (1860:105) described _P. aztecus_ he did not designate a type or type locality. Osgood (1909:157) designated as lectotype the mounted specimen, in the Geneva Museum, which has the skull inside and of which Saussure figured the molar teeth. Osgood (_loc. cit._) examined one of the three specimens (No. 3926 USNM) that Saussure used in describing _P. aztecus_ and found that it agreed "in every respect with recently collected specimens from Mirador, Veracruz, which, in the lack of exact knowledge, may be assumed to be the type locality, as it is certain that some at least of Saussure's specimens were taken near there." [Illustration: FIG. 1. Two species of _Peromyscus_. 1. _P. boylii ambiguus_ 2. _P. boylii beatae_ 3. _P. boylii levipes_ 4. _P. aztecus_ (triangles)] Osgood regarded _P. aztecus_ as a subspecies of _P. boylii_ because of the resemblance between _aztecus_ and _P. b. evides_, but _evides_ is far removed geographically (occurring only in western México) from _aztecus_, and is smaller. _P. aztecus_ is larger than any known subspecies of _P. boylii_, and is not known to intergrade with _P. b. levipes_ or _P. b. beatae_ (with which _aztecus_ occurs sympatrically at Jalapa, Veracruz), the two subspecies of _boylii_ that are found nearest the geographic range of _P. aztecus_. Also, as mentioned previously, _aztecus_ possesses distinctive characters that distinguish it from all subspecies of _boylii_. For these reasons I regard _aztecus_ as a distinct species.