A female obtained on June 26 carried eight embryos that measured 26, and was in process of seasonal molt. Testes of a male shot on May 20 measured 35. Order Rodentia ~Eutamias minimus pallidus~ (J. A. Allen, 1874) Least Chipmunk _Specimens examined_ (31).--2 mi. N, 5 mi. W Ludlow, 15; NW 1/4 sec. 15, R. 5 E, T. 22 N, 2; 2 mi. S, 3-1/4 mi. W Ludlow, 2; NW 1/4 sec. 32, R. 1 E, T. 20 N, 1; 9 mi. S, 7 mi. W Reva, 1; 10 mi. S, 5 mi. W Reva, 9; NE 1/4 sec. 8, R. 8 E, T. 16 N, 1. The least chipmunk is common in the buttes and associated badlands where it most frequently inhabits rocky areas. Visher (1914:88) reported _E. minimus_ from Harding County ("abundant in badlands"), but his paper has been overlooked by most subsequent workers. Visher's mention of a chipmunk from the mouth of the Moreau River in north-central South Dakota, incidentally, would seem to be in error, as would the report by Over and Churchill (1945:28) of _Eutamias_ inhabiting "... thickets along the Little Missouri River of Harding County." Females evidently bear but one litter annually (in late May) in northwestern South Dakota and young are weaned by the latter part of June. Females taken on May 15 and 19 carried embryos (five measuring 30 and three measuring 28, respectively). A lactating female with five placental scars was obtained on May 24, but eight adult females taken after June 23 previously had weaned young. Juveniles were collected on June 24 and 25. Testes of two adult males collected in mid-May measured 11 and 18, but males taken in summer had much smaller testes. In late spring, most adult least chipmunks molt from the worn, drab-gray pelage of winter to a brighter, more tawny summer pelage, but molt in a few females, perhaps originally delayed by reproductive activity, continues well into the summer months. Of seven specimens taken between May 15 and 24, two (one male and one lactating female) were in an early stage of molt, whereas the remainder were in winter pelage. Nine specimens (four females and five males) taken in mid- and late June were molting, but two females collected then were in winter pelage, and three animals, two males and a female, had completed molt to summer pelage. One adult female taken on August 5 had yet to complete molt to summer pelage. In our material, the first indication of molt from winter to summer pelage appears on the top of the head and the cheeks. Thereafter, molt proceeds posteriorly over the shoulder region and more or less evenly along the back and sides. In two specimens, small patches of molt preceded the general molt line. Molt on the venter apparently begins after molt on the dorsum approaches completion, but we could discern no definite pattern; on four specimens, hair was being replaced on the venter in scattered patches.