Fossiliferous strata exposed along the northwestern flank of Dry Mountain on the 111 Ranch, 27 km southeast of Safford, Graham County, Arizona, were dated by means of isotopic methods and the magnetic-reversal time scale. A 100-m fossiliferous interval contains the later part of the Gauss Chron and the early part (pre-Olduvai) of the Matuyama Chron. Zircon fission-track ages on an ash bed just below the Gauss-Matuyama boundary support this identification. Owing to these age constraints, the fossil mammals at the 111 Ranch locality are all constrained within the later part of the Blancan Mammal Age rather than the latest Blancan and early Irvingtonian, as previously stated in the literature. They are, therefore, synchronous with certain localities (Wolf Ranch and California Inst. Technology) in the Saint David Formation of the San Pedro Valley 130 km to the southwest of Dry Mountain. The horse Nannippus persisted in the 111 Ranch locality at least until the end of the Gauss Chron, in confirmation of data from the San Pedro Valley. A diverse group of equine and asinine equids is evident in the 111 Ranch fauna, which argues for an early divergence of these major branches of horse phylogeny. The 111 Ranch fauna records the first appearance of three neotropical immigrants in the southwest, including the sloth Glossotherium, the glyptodont Glyptotherium, and the capybara Neochoerus. A palearctic immigrant, the vole Synaptomys, is also present.