Diatoms, magnetic susceptibility, organic content, and14C ages of sediments from a 26-m core suggest that Lake Chalco, in the southern part of the basin of Mexico, went through a series of major fluctuations during the late Pleistocene and the Holocene. Before ca. 39,00014C yr B.P. the lake was very deep (about 8–10 m), alkaline, and saline. It then became shallow (<2 m) for most of the time between ca. 39,000 and 22,500 yr B.P. Chalco deepened to about 4–5 m about the time of a major eruption of nearby Popocatepetl volcano ca. 22,000 yr B.P. The lake remained relatively deep and fresh until ca. 18,500 yr B.P., when lower levels and alternating acidic to freshwater conditions were established. After 14,500 yr B.P. lake level rose slightly, but by ca. 10,000 yr B.P. Chalco became very shallow (<2 m), remaining as a low, alkaline saline marsh until ca. 6000 yr B.P. This period corresponds with the Playa cultural phase, during which the earliest human settlements in the basin were established. After ca. 6000 yr B.P. Chalco became a fresh to slightly alkaline shallow lake a few meters deep.