Since the name Thomomys clusius was first proposed in 1875, it has been applied and misapplied to a variety of different pocket gophers. Study of specimens and chromosomal material of pocket gophers collected from southern Wyoming permits the recognition of Thomomys clusius as a distinct species within the Thomomys talpoides complex. Thomomys clusius is a small, pale-colored animal with 2n = 46 chromosomes. It does not interbreed with T. talpoides ocius where the two are found together. The known range of T. clusius is almost entirely within that of T. t. ocius, but the two occur in different habitats. Also, T. clusius is chromosomally and morphologically distinct from the pale-colored populations of T. talpoides of central Wyoming to which Long (1965) applied this name.