This account represents a thorough update of Mammalian Species 210 on the subject species by A. V. Linzey, incorporating 102 relevant references published since 1983. Synaptomys cooperi , the southern bog lemming, is an arvicoline rodent. This compact snub-nosed rodent has a thick grayish coat, small eyes and ears, gray or silver venter, and a stubby tail as long as the hind foot. It occurs from Canadian maritime provinces to southeastern Manitoba, southward to Arkansas and northeastern North Carolina. This cold-adapted species lives in mesic grasslands in the Great Plains states, in wet meadows in some eastern states, and often in wet coniferous and other forest types in the north. Its presence often can be detected by distinctive green droppings. One of two species in the genus, S. cooperi is sometimes difficult to capture and study, leading to its status of conservation concern in some states and provinces.