Fragments of a bone described here as the probable rib of a large plesiosaur were recovered from the lowest septarian concretion zone in the Blue Hill Shale Member (Middle Turonian) of the Carlile Formation in southwestern Mitchell County, Kansas. If this identification is correct, the specimen represents the first documented occurrence of plesiosaur remains in the Blue Hill Shale and adds to the poorly known vertebrate fauna from this member. The rib, however, is non-diagnostic other than its large size, and could represent either a large elasmosaur or a pliosaur such as Brachauchenius lucasi.