The diet of African hippopotamids can be documented through δ13C analyses of enamel and other tissues. Analysis of a 10-million-year sequence of hippopotamids in and near the Lake Turkana Basin of northern Kenya shows that hippos have included a substantial fraction of C3 vegetation in their diets since the late Miocene when C4 vegetation first appears in hippo diet as a measurable fraction. The C4 component of vegetation becomes dominant (>50%) by Upper Burgi time (c. 2 million years ago) but does not reach 100% for all individuals. It is therefore not unexpected that the δ13C values of modern hippopotamids show a higher fraction of dietary C3 biomass than has been estimated from traditional observations. Analysis of δ18O of hippos from different stratigraphic levels shows no systematic trend over time; the average value for fossil hippos over the last 10 million years is similar to that of modern hippos from the Omo River system.