Two species of lagomorphs are represented in the Diamond O Ranch local fauna of southwestern Montana. Mytonolagus ashcrafti, new species, is the most common mammal at Diamond O Ranch, and is comparable in evolutionary grade to Mytonolagus wyomingensis Wood, 1949, and more advanced than either Mytonolagus petersoni Burke, 1934, or Mytonolagus near M. petersoni. A second species of lagomorph is represented by only a few specimens from near the base of the Diamond O Ranch section. The teeth of this form tentatively assigned to Palaeolagus Leidy, 1856, are significantly more hypsodont than those of M. ashcrafti. The Diamond O Ranch local fauna falls very near the Duchesnean—Chadronian boundary in one of the most poorly documented temporal intervals during the entire Cenozoic of North America.