_Comparisons._--For comparison with the subspecies newly described below from northern Colorado see the account of that subspecies. The subspecies _M. p. aztecus_ has been compared with _M. p. modestus_ by Hall and Cockrum (1952:308) who reduced _aztecus_ to subspecific rank. Although _aztecus_ is separated by approximately 100 miles from _modestus_, and although no proof of intergradation is available, my studies of variation in this species lead me to agree with Hall and Cockrum that "the morphological differences between the two kinds of animals are of the degree and kind that separate subspecies, rather than species." A more adequate series of adults of _aztecus_ is needed to clarify even the subspecific differences between _aztecus_ and _modestus_. _Measurements._--Averages, extremes, and standard deviations of a number of series are included in Table 1 in order to facilitate comparisons between different subspecies. _Remarks._--The dividing line between _M. p. modestus_ and the subspecies to the north on Figure 2 is drawn somewhat arbitrarily because few specimens are available from this area. Actual intergradation, in the form of a geographically intermediate population also morphologically intermediate between these two subspecies, is lacking. However, in most populations of both subspecies some individuals are intermediate between the two subspecies or even more like the other subspecies than the one to which they are referred. Warren (1942:226) states that _modestus_ has been recorded from Lake County, although no reference to a specimen is given. Bailey (1900:21) cites Twin Lakes, in Lake County. That county is near the dividing line as I have drawn it, and therefore specimens from Lake County would be of special interest. An isolated colony of _modestus_ occurs at San Rafael in Valencia County, New Mexico (Bailey, 1932:201). A hiatus of approximately 150 miles separates that colony from the southernmost locality shown in Figure 2. A single specimen, the skin of an immature _Microtus_ without skull, from Trinchera, Colorado, taken by L. R. Hersey in 1912, is in the Colorado Museum of Natural History. No species of Microtine has been recorded from within 50 miles of this locality. The specimen is seemingly more like _M. pennsylvanicus_ than any other species of _Microtus_. This locality is represented in Figure 2 by a question mark. Ecologically _M. montanus fusus_ Hall and _M. longicaudus mordax_ (Merriam) in the same region occupied by _modestus_ seem to be more montane than _modestus_. It favors lush grass on the wet floors of alluvial valleys, and also irrigated areas such as that at Manassa.