The accidental discovery in July 1996 of the Tocuila mammoth site in the eastern Valley of México was followed by intensive scientific excavation, which yielded remains of at least seven individuals of Mammuthus columbi in an area measuring only 28 m2. The high density of mostly disarticulated mammoth bones identified 3 m below the surface in a massive, 130-cm-thick lahar deposit of sandy ash and rounded pumice fragments is unusual and suggests a high-energy depositional environment. Samples of charcoal found within this deposit yielded radiocarbon ages ranging between 10 220 ± 75 and 12 615 ± 95 yr B.P. The lahar was studied in greater detail in order to establish its provenance. Determination of grain-size distribution and magnetic properties of the deposit, as well as petrographical, mineralogical, and geochemical analyses of pumice clasts, all indicate that the lahar was derived from volcanic fallout deposits that originated at Popocatépetl volcano, located 50 km south-southeast of the site. Popocatépetl underwent a major phreato-Plinian eruption, dated ca. 14 000 yr B.P., at the end of the last major glaciation. Dispersal of the fallout was mostly toward the northwest and north; deposits were found on the slopes of Tláloc mountain (4150 m), 20 km southeast of Tocuila. At the beginning of deglaciation and the onset of more humid conditions ∼3000 yr later, lahars derived from the fallout deposits picked up the bones of already dead animals and then deposited them on the shores of Lake Texcoco and/or simply covered bones without major displacement.