_Measurements._--The average and the extremes for some measurements of 34 males and females, 27 from the type locality and 7 from other localities in the range assigned to this subspecies, are as follows: total length, 165 (146-186); length of tail, 44.2 (35-55); hind foot, 19.6 (17-21); condylobasilar length of the skull, 25.5 (24.0-27.5); zygomatic breadth, 15.6 (14.7-16.6); alveolar length of upper molar tooth-row, 6.6 (6.2-7.0); prelambdoidal breadth, 8.8 (8.1-9.5); lambdoidal breadth, 12.0 (11.2-12.8). As an indication of variability and for comparison with other series the coefficient of variability and two times the standard error of the mean for each measurement in this series are included in Table 1. The averages for some measurements of 27 topotypes are as follows: total length, 162; length of tail, 45.5; hind foot, 19.9; condylobasilar length, 25.6; palatilar length, 14.0; molar series, 6.6; alveolobasilar length, 14.9; zygomatic breadth, 15.6; interorbital breadth, 3.5; lambdoidal breadth, 12.1; prelambdoidal breadth, 8.9; depth of braincase, 7.8. _Discussion._--Three species of _Microtus_ were collected by James W. Bee at the type locality. _Microtus montanus codiensis_, _Microtus longicaudus mordax_, and _Microtus pennsylvanicus modestus_ were taken in the same runways in the same meadow, at the same time. _Microtus ochrogaster haydeni_, although not taken at this locality, occurs in the Big Horn Basin. These four species differ in their geographic ranges, being largely allopatric, except _M. montanus_ and _M. longicaudus_ which are sympatric. Although the different species have ecological preferences and habits which differ, several species of _Microtus_ may occur together in local areas such as the above. Certain of the characteristics of _M. m. codiensis_ are intermediate between those of the species _M. montanus_ on one hand and those of the other three species on the other hand. Could interspecific hybridization between "good species" of _Microtus_ take place in nature and possibly alter the characteristics of a local population? _Specimens examined._--Total, 50, distributed as follows (abbreviations for collections are given in the account of _M. m. nanus_; localities that are not represented in Fig. 1 because overlapping or crowding of the symbols would result are Italicized): MONTANA: _Carbon Co._: Beartooth Mountains, 2 (USBS); _Beartooth Lake_, 1 (USBS). WYOMING: _Park Co._: Black Mountain, head of Pat O'Hara Creek, 3 (USBS); 13 mi. N, 1 mi. E Cody, 5200 ft., 1; SW slope Whirlwind Peak, 9000 ft., 1; _5 mi. N Cody, 6300 ft._, 1 (USBS); 3⅕ mi. E, ⅗ mi. S Cody, 31; Ishawooa Creek, 6300 ft., 2 (USBS); _Valley_, 1 (USBS); Needle Mountain, 10,500 ft., 4 (USBS). _Hot Springs Co._: 3 mi. N, 10 mi. W Thermopolis, 4950 ft., 3. =Microtus montanus zygomaticus=, new subspecies