A Proterozoic map-scale sheath fold is present in the crystalline core of the Tobacco Root Mountains. The fold has distinctive metamafic and associated metasedimentary rocks of the Spuhler Peak Metamorphic Suite in its core and quartzofeldspathic gneisses of the Indian Creek and Pony–Middle Mountain Metamorphic Suites on its outer flank. Fabric and structures throughout the range relate geometrically to this sheath fold and together demonstrate the character of ductile deformation during the 1.72–1.78 Ga Big Sky orogeny in the Tobacco Root Mountains. Intense, progressive, noncoaxial strain either transposed or formed compositional layering, mineral-alignment foliation, and lineation and generated outcrop-scale sheath folds, cylindrical folds with axes dispersed in the plane of foliation, and larger curtain folds. While it can be demonstrated that some gneissic fabrics and folds in the Indian Creek and Pony–Middle Mountain Metamorphic Suites formed in a much earlier phase of deformation that predates the intrusion of a suite of metamorphosed mafic dikes and sills, the Big Sky orogeny effectively reoriented preexisting structures and fabrics into a single Proterozoic pattern. It is most likely that tectonic juxtaposition of the Spuhler Peak Metamorphic Suite with the quartzofeldspathic gneiss suites was an integral part of this shear strain. Mesoscopic asymmetric folds record the displacement of the Spuhler Peak Metamorphic Suite down and to the north relative to the Indian Creek and Pony–Middle Mountain Metamorphic Suites during the formation of the map-scale sheath fold. “Unfolding” this sheath fold demonstrates that the Indian Creek and Pony–Middle Mountain Metamorphic Suites are lateral equivalents but leaves undetermined whether the Spuhler Peak Metamorphic Suite was juxtaposed above or below the quartzofeldspathic gneisses during Big Sky deformation.