Four specimens of a small fossil fish were collected from the Eocene Mahenge site of Tanzania. The specimens, preserved as part and counterpart natural moulds, are identified, predominantly based on the structure of the caudal skeleton, as members of the Characiformes, probably the sister group to the living African Alestidae. The area just behind the skull, in the two specimens that include this area, is distorted, and therefore it is difficult to identify the bones of the Weberian apparatus, although that structure does appear to be present. The fossil record of characiforms includes few articulated skeletons, with only one other African species previously reported from much younger deposits. The new specimens from Mahenge are described here as a new genus and species, Mahengecharax carrolli.