While working upon material obtained recently in the Gulf of Aden I found it necessary to examine the collection of Hammerhead Sharks in the British Museum (Nat. Hist.). The results of this study were somewhat surprising and have necessitated the publication of this paper. During my work in the field I distinguished three species which I named Sphyrna zygaena, S. mokarran and S. tudes, in accordance with the previously accepted definitions of these forms, but it now appears that not one of these names was correctly applied. The position can best be made clear by discussing briefly each of these in turn.