Specimens from the northwestern part of the range of _dychei_ (Wyoming, Montana and western South Dakota), like those from western Nebraska, average slightly paler dorsally than topotypes and other specimens from eastern Kansas and Nebraska (a few approach _aztecus_ in this regard), but do not otherwise differ. Most specimens from northern Colorado, southwestern Nebraska (Hitchcock and Dundy counties) and western Kansas average slightly paler than typical specimens and have longer rostra, approaching _aztecus_ in these particulars, but have the shorter ears and shorter tail of _dychei_. In general, these intergrades resemble _dychei_ to a greater degree than _aztecus_ and are accordingly assigned to the former. One exception is a series from Muir Springs, 2 mi. N and 2-1/2 mi. W Ft. Morgan, Colorado. Specimens in this series approach typical _dychei_ in color, but resemble _aztecus_ in having long ears and long rostra (average 15.3 and 7.5, respectively, in 13 adults). The specimens from Muir Springs resemble _aztecus_ to a greater degree than _dychei_, but are assigned to the latter because specimens from farther west and farther south in Colorado are assignable to _dychei_. Howell (1914:31) earlier noted that specimens from northern and central Colorado were intergrades between the two subspecies. The geographic range occupied by _R. m. dychei_ (from east of the Mississippi River in Illinois and Wisconsin to the foothills of the Rockies) is large (although not so large as that currently ascribed to _R. m. megalotis_, which ranges from southern British Columbia to central México). Most other small rodents that occur in the same geographic area occupied by _dychei_ are represented there by at least two subspecies, a dark one in the east and a pale one in the west. Eastern populations of _dychei_ have, it is true, somewhat darker upper parts than mice from western localities, but the differences are slight; also, judging from the literature, the "pectoral spot" is more common in eastern mice. It should be noted that _R. m. dychei_ probably has extended its range both eastward and westward in the last century as a result of agricultural practices--clearing of land in the east and irrigation in the west. _Specimens examined._--1145, as follows: COLORADO. _Adams County_: South Platte River, 5 mi. N Denver, 1; 3 mi. S, 1 mi. W Simpson, 1. _El Paso County_: 5 mi. E Payton, 1; 4 mi. S maingate of Camp Carson, 1. _Larimer County_: 3 mi. N Loveland, 1; 9-1/4 mi. W, 1/2 mi. N Loveland, 5600 ft., 1; 16 mi. W Loveland, 6840 ft., 1; 3-1/2-4-1/2 mi. W Loveland, 5030 ft., 7; 6 mi. W, 1/2 mi. S Loveland, 5200 ft., 14; 7 mi. W, 2-1/2 mi. S Loveland, 5370 ft., 1. _Morgan County_: Muir Springs, 2 mi. N, 2-1/2 mi. W Ft. Morgan, 21. _Washington County_: Cope, 6. _Yuma County_: 1 mi. W to 1 mi. E Laird, 6.