Recent advances in our understanding of the Plio-Pleistocene sedimentary sequence northeast of Lake Turkana in northern Kenya has allowed a revision of the lithostratigraphic nomenclature and a more complete description of the deposits. As redefined, the Koobi Fora Formation encompasses the entire Plio-Pleistocene sedimentary sequence, and is subdivided into eight members. These members are delineated using volcanic ash horizons, which can be identified uniquely on the basis of chemical composition. The revised stratigraphy resolves all earlier conflicts with the biostratigraphic zonations based on vertebrate and molluscan studies. The sedimentary sequence of the Koobi Fora Formation attains an aggregate thickness of 560 m and spans the period from about 4.3 to 0.6 Ma. Detailed correlations have now been established to relate this sequence to other deposits within the Turkana Basin and to those at Hadar and in the Gulf of Aden.