The Madison Group (Mississippian) of the Sun River Canyon area, northwestern Montana, is subdivided into 2 newly named and defined formations - the Allan Mountain Limestone and Castle Reef Dolomite. The Allan Mountain Limestone is composed of 3 unnamed members: the lower member, middle member, and upper member. The Castle Reef Dolomite consists of an unnamed lower member and the redefined Sun River Member. The recognition of 4 faunal zones in the Madison permits tentative correlation of these rocks with rocks of Mississippian age in central and southwestern Montana and in Alberta, Canada. Formational boundaries and lithologic types recognized in southern Montana can not be consistently followed in the Sawtooth Range. The faunas indicate that the Allan Mountain Limestone is approximately the same age as the Lodgepole Limestone of southwestern Montana and that the Castle Reef is approximately the same age as the Mission Canyon Limestone. The top of the middle member of the Allan Mountain Limestone is approximately equivalent to the boundary between the Banff and Rundle formations of Alberta. The Ca/Mg molar ratio substantiates field determinations of the lithologic composition and boundary between the 2 formations. Local correlation of certain units are indicated by the molar ratio.