The early hominids of East Africa were dated by determining the ages of tuff beds at the sites. Despite much research using palaeomagnetic and K/Ar-dating techniques, some of those ages are still controversial 1,2. To obtain independent age estimates for these tephra layers, we have examined cores from DSDP Sites 231 and 232 in the Gulf of Aden (Fig. 1a) which consist mainly of calcareous nannofossil ooze, but also contain rare tephra horizons3 dated by interpolation from the established nannofossil stratigraphy (Fig. 1b). Chemical analysis confirms that the identity and sequence of these horizons is the same as that at the East African sites. We conclude that the age of the Tulu Bor Tuff is <3.4 Myr and hence that the Hadar hominid specimens are also ??3.4 Myr old. ?? 1985 Nature Publishing Group.