Twelve species of marine shell were transported in significant quantities from the Red Sea to the trade centre of Harlaa in eastern Ethiopia between the eleventh and early fifteenth centuries AD. Initially, it was thought that species such as the cowries were imported from the Indian Ocean. Subsequent research has found that all were available from the Red Sea and the Gulf of Aden, c. 120 km east of Harlaa. This suggests that a hitherto largely unrecognised source of marine shells was available, and the Red Sea might have supplied not only the Horn of Africa, but other markets, potentially including Egypt, and from there, elsewhere in North Africa and ultimately West Africa via trans-Saharan routes, as well as Nubia and further south on the Nile in the Sudan, the Arabian Peninsula, and the Arabian/Persian Gulf. This is explored with reference to the shell assemblage from Harlaa, and selected shell assemblages from elsewhere in the Horn of Africa, and trading centres on the Red Sea.