This woodrat is relatively uncommon, yet widely distributed, in northwestern South Dakota. The species probably occurs throughout the rocky areas in the hills and buttes of Harding County, and also frequents abandoned or little-used buildings and feed stations for livestock. All of our specimens were trapped in rocky habitats, but in many such places we trapped unsuccessfully for _Neotoma cinerea_, even though some sign of its presence frequently was evident. Five of our seven specimens (all taken late in June or early in July) are young of the year in grayish pelage. An adult male trapped on July 14 had testes that measured 14. Over and Churchill (1945:40) mentioned a specimen, which they referred to the subspecies _N. c. cinerea_, that "probably came from the Slim Butte area of Harding County." Visher (1914:89) recorded the species as "plentiful and general" in the county. ~Microtus ochrogaster haydenii~ (Baird, 1858) Prairie Vole _Specimens examined_ (40).--NE 1/4 sec. 22, R. 1 E, T. 23 N, 1; NW 1/4 sec. 15, R. 5 E, T. 22 N, 1; 2 mi. N, 5 mi. W Ludlow, 4; 4 mi. S, 7 mi. W Ladner, 19; 1/2 mi. W Reva, 2; SW 1/4 sec. 30, R. 7 E, T. 18 N, 3; 10 mi. S, 5 mi. W Reva, 10. The prairie vole is the most abundant of the three microtines that are known from Harding County. Our specimens came primarily from areas of relatively dense grasses, such as those found in stream bottoms and along fencerows. At two places, along a fencerow 1/2 mi. W Reva and in Deer Draw, 10 mi. S and 5 mi. W Reva, _Microtus ochrogaster_ and the meadow vole, _M. pennsylvanicus_, were trapped together. Nine females taken late in May and in June carried an average of 4.4 (3-6) embryos that averaged 14.8 (4-25) in crown-rump length. Each of two lactating females taken in late May had six placental scars. Testes of seven adult males taken in May and June averaged 13.1 (12-16) in length. ~Microtus pennsylvanicus insperatus~ (J. A. Allen, 1894) Meadow Vole _Specimens examined_ (14).--1/2 mi. W Reva, 1; 10 mi. S, 5 mi. W Reva, 12; 7 mi. S, 4-1/2 mi. E Harding, 1. This vole evidently is limited in Harding County to habitats of dense grass and forbs adjacent to water. We failed to trap the species in some areas that appeared to be suitable for occupancy. Two females, collected on May 15 and June 2, contained three and six embryos, respectively, that measured 15. One female with five placental scars (May 15) and two with six (May 16, June 2) also were trapped, and a lactating female was captured on June 20. Testes of two adult males taken in spring (May 18 and June 2) measured 15, and those of one weighing 45.6 grams that was taken early in June measured 17. ~Ondatra zibethicus cinnamominus~ (Hollister, 1910) Muskrat The muskrat is common in Harding County. Half a century ago Visher (1914:89) noted that it was "Fairly plentiful along the streams having deep permanent 'holes'."