 The road turns north just before crossing the county line from Phillips County into Blaine County, and subsequently enters the Fort Belknap Indian Reservation where it remains for the rest of its journey.After passing the small community of Hays on its western edge, the road continues north on a mostly straight route before turning northwest and enters Fort Belknap from the south, before a final turn to the northeast joins MT 66 to US 2 at a rest area.Before receiving its current designation in 1978, MT 66 was designated as Montana Secondary Highway 376 (S-376).