The new species, Formica gynocrates, is described and illustrated from all the castes. This slave-making species is a member of the sanguinea group and is most similar to F. pergandei. The type locality is the E. S. George Reserve, Livingston County, Michigan. Specimens were also examined from North Dakota, Wyoming, and Colorado. A key is provided for the separation of the described North American members of the sanguinea group. The most commonly associated slave species is F. vinculans Wheeler, a member of the neogagates group. It is our opinion that F. vinculans is a valid species and not a synonym of F. neogagates, as has been previously supposed
