_aztecus_, are intergrades between it and _dychei_, as are two specimens from El Paso County, to the north, which resemble _aztecus_ in color but resemble _dychei_ in other characters and are tentatively assigned to the latter. Specimens from southwestern Kansas and adjacent Oklahoma, herein referred to _aztecus_, also are intergrades with _dychei_. Individuals from Meade County, for example, are intermediate on the average between typical specimens of the two subspecies in color of upper parts (if anything, nearer _dychei_), resemble _dychei_ in length of ear, but resemble _aztecus_ in length of tail and rostral proportions (consequently also in length of skull). Although a case could be made for assignment of the specimens from Meade County (and elsewhere in southwestern Kansas) to _dychei_, they are, everything considered, nearer _aztecus_, to which subspecies they have been assigned consistently since first reported from the area by Hill and Hibbard (1943:24). Of two specimens examined from 10 mi. S and 1 mi. W Gruver, Hansford Co., in the Panhandle of Texas, the one adult is clearly assignable to _aztecus_ as is the specimen from 9 mi. E Stinnett, Hutchinson Co., Texas, that was referred to _dychei_ by Blair (1954:249). _Reithrodontomys megalotis aztecus_ has had a rather unstable taxonomic history. Allen, who originally named the subspecies (1893:79), regarded it two years later (1895:125) as a synonym of _R. m. megalotis_, the subspecies with geographic range to the south and west of that occupied by _aztecus_. Howell (1914:30) recognized _aztecus_ as valid, but he, too, questioned its distinctness from _megalotis_ in a later paper (1935:144). Hooper (1952:218), the most recent reviewer, supported the validity of _aztecus_ because specimens available to him averaged "distinctly larger in skull length and size of brain case" than specimens of _megalotis_. Our comparisons of typical specimens of _aztecus_ with specimens of _megalotis_ from southern New Mexico and southwestern Texas confirm Hooper's observations and indicate also that _aztecus_ has a longer rostrum and slightly longer ear. _Specimens examined._--205, as follows: COLORADO. _Alamosa County_: Medano Ranch, 15 mi. NE Mosca, 16 (USNM). _La Plata County_: 1 mi. NW Florida, 6700 ft., 1; Florida, 6800 ft., 1. _Las Animas County_: 1 mi. S, 7 mi. E Trinidad, 2. _Montezuma County_: 1 mi. W Mancos, 5; north end, Mesa Verde Nat'l Park, 7000 ft., 3; Far View Ruins, Mesa Verde Nat'l Park, 7700 ft., 3; Park Point, Mesa Verde Nat'l Park, 8525 ft., 2; within 3 mi. Rock Springs, Mesa Verde Nat'l Park, 7500-8200 ft., 6. _Prowers County_: Lamar, 2. _Rio Grande County_: Del Norte, 1 (USNM).