ABSTRACT. The hominine cranium KNM-ER 1813, from the late Plio/Pleistocene of Koobi Fora, has been regarded recently by some authors as a female of Homo habilis LEAKEY, ToaIAS, and NAPIER, 1964 and by others as an enigma. Reassessment of its cranial morphology, dental metrics, proportions, and a new detailed determination of its sex indicates that it does not conform with the diagnosis for H. habilis, and is probably a male. It is sympatric with H. habilis yet shows more primitive features and rather a closer affinity to the smaller, more primitive chronospecies H. antiquus FERGU-SON, 1984, and is thus the first, nearly complete skull of our oldest known human ancestor.