_Comparisons._--From _Synaptomys cooperi gossii_ (specimens from eastern Nebraska and eastern Kansas, including one topotype), _S. c. relictus_ differs in: Dorsal coloration, in comparable pelages, darker, venter lacking buffy tinge; skull averaging larger in all cranial dimensions (except rostral length, which is approximately the same), especially breadth of upper incisors and length of molariform tooth-rows; nasals broader anteriorly; auditory bullae nearly equal in size, thus relatively smaller. From _Synaptomys cooperi paludis_ (holotype and paratypes) of Meade County, Kansas, _S. c. relictus_ differs in: Skull averaging smaller in all cranial measurements except rostral length and breadth of upper incisors, which are approximately the same, and breadth of nasals, length of incisive foramina and length of molariform tooth-rows, which measure more; nasals relatively (48 per cent of length of nasals) as well as actually broader anteriorly; anterior border of zygomatic plate more concave; auditory bullae smaller; infraorbital foramina larger when viewed anterolaterally. _S. c. relictus_ closely resembles _S. c. paludis_ in color and external proportions. _Remarks._--The total population of _Synaptomys cooperi relictus_ may be small because the only suitable habitat known to me for these mice is the dense, grassy area, approximately 100 yards wide and one mile long, around some of the rearing ponds and along the creek at Rock Creek Hatchery. It has been taken there in association with _Cryptotis parva parva_, _Blarina brevicauda carolinensis_, _Reithrodontomys megalotis dychei_, _Peromyscus maniculatus nebrascensis_, _Microtus ochrogaster haydenii_, and another relic, _Microtus pennsylvanicus finitis_. All specimens of the newly named bog lemming are from the border zone between the wet-substrate habitat of _M. p. finitis_ and the drier habitat occupied by _M. o. haydenii_. Approximately 3000 trap nights produced the four known specimens. _S. c. relictus_, like _S. c. paludis_, represents a relict population of the more southwesterly distribution of the subgenus _Synaptomys_ during Wisconsin and post-Wisconsin times. Additional relict populations likely will be found in the eastern Great Plains. The new subspecies is intermediate in some features between _paludis_ and _gossii_. The type locality is separated from that of _paludis_ (14 mi. SW Meade, Meade County, Kansas) by a distance of approximately 220 miles over habitats largely unsuitable for bog lemmings. The nearest locality of record for S. c. _gossii_ to the east of the type locality of _relictus_ is at Hunter, Mitchell County, Kansas (see Cockrum, Univ. Kansas Publ., Mus. Nat. Hist., 7:196, 1952), approximately 200 miles distant. The locality of record of _gossii_ in Nebraska nearest to the type locality of _relictus_ is even farther eastward--1 mi. N Pleasant Dale, Seward County (KU 50188). _Specimens examined._--Four, from the type locality (KU 51617, 72601-03).