The western harvest mouse was taken commonly in stands of tall grasses and forbs, particularly along roadways and fencerows. Occasional individuals were trapped in areas of mixed shrubs and grasses. Four pregnant females taken in late June carried the following number of embryos (crown-rump lengths in parentheses): seven (4), six (5), six (10), five (4). Three adult males taken in the same period had testes that measured 7, 7, and 8, whereas those of two May-taken males measured 12 and 6. Molt from winter to summer pelage was in progress, from anterior to posterior, on both the dorsum and venter of many May- and June-taken animals. Some individuals had completed molt, or had but a small patch of winter pelage remaining on the rump, as early as the last week in June. ~Reithrodontomys montanus albescens~ Cary, 1903 Plains Harvest Mouse _Specimens examined_ (3).--2 mi. N, 5 mi. W Ludlow, 2; 1/2 mi. W Reva, 1. This harvest mouse is uncommon in northwestern South Dakota, although the species probably occurs sparingly in upland grassy habitats throughout Harding County. Our specimens, along with one in the collections of the University of Michigan Museum of Zoology, not previously reported, from 11 mi. S Mandan, Morton Co., North Dakota, represent the northernmost known records of this mouse. A young adult female, obtained on June 21, carried three embryos that measured 17 and was in summer pelage; an adult male taken on June 27 still was in a worn winter pelage. At the locality 1/2 mi. W Reva, where traps were set in sparse to relatively lush grassy areas along South Dakota Highway 20, the following small mammals were taken in the same trapline (or adjacent lines) in which one plains harvest mouse was captured: _Spermophilus tridecemlineatus pallidus_, _Reithrodontomys megalotis dychei_, _Peromyscus maniculatus nebrascensis_, _Microtus ochrogaster haydenii_, and _Microtus pennsylvanicus insperatus_. ~Peromyscus leucopus aridulus~ Osgood, 1909 White-footed Mouse Seven adults of this woodland inhabitant were trapped along shrub-covered banks of the spring-fed stream and small impoundment in Deer Draw of the Slim Buttes (10 mi. S and 5 mi. W Reva). Deciduous trees grew in the bottom of the draw, but the slopes above supported ponderosa pine and juniper. No white-footed mice were found along the generally treeless tributaries of the Moreau and Grand rivers to the east of Slim Buttes nor were these mice found along the Little Missouri River or in likely-looking habitat in the North Cave Hills. The _P. leucopus_ of Deer Draw likely represent, therefore, an isolated segment of a formerly much more broadly distributed population of white-footed mice on the Northern Great Plains in post-Wisconsin times. Other such populations may exist in Slim Buttes and perhaps elsewhere in the county. _Zapus hudsonius_ and _Microtus pennsylvanicus_ were trapped in Deer Draw in association with white-footed mice.