The molting cycle of the yellow-pine chipmunk, Eutamias amoenus affinis, in the Cascade Mountains of Washington was studied. Details of pattern and sequence were obtained from animals dyed black before each molt and from young born and raised in captivity. Field data came. from the live-trapping of about 600 chipmunks. Except for the tail and rump, which shed only once, there were two complete molts each year, one beginning in May (adults) or June (young) and the other in September. On dyed chipmunks, 44 more or less distinct molting areas were observed. Some corresponded with color patterns, but the conspicuous dorsal stripes did not have separate molts. In spring, young animals began to molt on the head but very soon new hair also began to appear on the body and limbs. Overall progress on the head was posterior. On the body the sequence moved up from the sides to form a saddle across the back, then forward over the shoulders and backward to the rump; ventrally the movement was from mouth to belly. The tail molted next, and the last of the body to be renewed was the rump. The limbs shed first on the toes, and front legs preceded the hind. The crown and the heels finished molting at about the same time as Lhe rump. Adults in their spring molt differed from young in three ways: (1 ) in adults, a patchy or spotted pattern preceded the overall molt on the back, (2) the main molt of the body moved from shoulders to rump following an early spotty period, and (3) adults took twice as long as young (about 120 days) to complete this molt. The fall molt moved rapidly forward from rump and belly in both young and adult. The crown and nose were last. Adult males in the population as a whole had peaks of molting in spring and summer in the order: (1) head, (2) shoulders, (3) back, (4) chest and then belly, (5) tail, and (6) rump. In adult breeding females the shedding of the body was delayed. In the whole population the spring molt took about 144 days, but if tail and rump are omitted the time was only 1 10 days. The fall molt (not involving tail and rump) took 93 days. The. entire cycle spanned approximately 168 days from 1 May until 15 October in 1947. Young chipmunks were later than adults in both spring and fall cycles. A complete summer pelage lasted only a week or two, but the winter coat was worn 8 to 11 months. The molting cycle was about 3 to 4 weeks later in 1946 than in 1947, possibly because of a difference in weather in the two years. The underfur on the back is 1.4 times as long (7.6 mm) and 2.7 times as dense (174 hairs per mm2) in winter than in summer. Appropriate temperature regulation and protective coloration in different seasons probably are the most compelling reasons for the two molts and two pelages each year.