This report summarizes the 1970 season of archaeological research conducted by the authors in the Ayacucho Basin of Peru. The most important work undertaken during the season were excavations into preceramic deposits in Pikimachay Cave. A radiocarbon date from the second lowest layer yielded a date of 17,650 B.C. +/-3000. The determination was on a sloth vertebra, which was associated with various stone tools of the Paccaicasa complex. Excavations at the nearby Puente site revealed a stratified sequence of 36 superimposed floors in a six meter deep refuse deposit spanning the period from about 7000 to 1700 B.C., as determined by radiocarbon dates. The relationship between Puente phase materials, which pre-date 7000 B.C., and Jaywa Phase materials, which appear shortly after that date, was clarified by excavations at Jaywamachay, where materials of the two phases were found in a stratified deposit and radiocarbon dates were obtained. Materials of the preceding Huanta phase were also present. The results of the various excavations provide a skeleton preceramic chronology for the Ayacucho Basin extending back to perhaps 21,000 B. C. Following the earliest (Paccaisa) phase, the Ayacucho phase began around 14,000 B.C., with the Huanta Phase beginning around 8800 B.C. The later preceramic chronology was also greatly clarified by these excavations. A good groundwork for further work was a major result of the season's work.