A series of 39 specimens from 1 mi. SSE Ameca, 4000 ft., Jalisco, are uniformly grayish-brown. This series averages grayer than paratypes of _allex_. There is little, if any, difference between the series from 1 mi. SSE Ameca and paratypes of _allex_ in external size of body, hind foot, length of ear, and size and conformation of the cranium. Populations from Ameca and vicinity might be expected to average considerably larger inasmuch as they occur at higher altitudes (see Bergman's Rule, p. 609) then the material from the lower coastal plains to the south in Colima and Michoacan, and at lower elevations in the west in Jalisco and Nayarit. The means of external and cranial measurements are not significantly different between the specimens from the highlands and those from the lowlands. In the area of Ameca where the two species _B. musculus_ and _B. taylori_ occur together, interspecific competition seems to have limited, perhaps even reduced, size of external and cranial parts of _taylori_ (see p. 660). In color, specimens from the northern part of the valley of the Rio Tepalcatepec (10 mi. S, 1 mi. W Apatzingan) in Michoacan resemble paratypes of _allex_. Intergradation probably occurs to the north with _analogous_. In the eight specimens from 13 mi. E and 1 mi. N Talpa de Allende, the skull, as reflected in occipitonasal length and zygomatic breadth relative to length of body, is larger than in other specimens here assigned to _allex_. The median part of the belly of the eight specimens is buff-colored rather than whitish-gray as in typical _allex_; the mid-dorsal region also averages darker than in any other specimens referred to _allex_. Additional specimens are needed from this and closely adjacent areas, especially to the west on the coastal plain, in order to determine more accurately the taxonomic status of the mice there. At present, it seems best to refer them to _allex_. Possibly the population represented by the eight specimens is an incipient subspecies. There is no evidence of hybridization or intergradation of populations of _B. t. allex_ with any population of _B. musculus_ where the two species occur together.