Fossiliferous middle Pleistocene sediments of the Tarija basin of southern Bolivia contain a classic Ensenadan land-mammal fauna. New carbon isotopic data reported here for 50 specimens of the grazing mammals Equus (horse) and Cuvieronius (mastodon), documented from eight stratigraphic levels at Tarija, vary significantly in the δ13C values of their teeth. The pattern of variation appears to reflect the proportion of C3 and C4 grasses eaten during colder (more C3) and warmer (more C4) times. Within age limits set by associated magnetostratigraphy, the cold periods can be correlated with particular even-numbered stages in the marine oxygen-isotope record, and the warm periods can be correlated with odd-numbered stages. The oldest fossil teeth analyzed from the Tarija section can thereby be assigned to stage 29, and the youngest to stages 17 or 15, that is; the teeth range in age from about 1.1 myr to as young as 0.7 myr. Based on correlation of the upper part of the Tarija beds to the isotopic stages, the upper limit of the Ensenadan land-mammal age is between 0.7 and 0.6 myr, which is younger than stated in most previous studies.