Polybius reports that the second Romano-Carthaginian treaty included a clause forbidding the Romans to pillage, trade, or found cities beyond the Fair Promontory (modern Cape Bon) and a mysterious place called Mastia Tarseion. On the assumption that Mastia Tarseion was located in southern Iberia, modern scholars have used the treaty as evidence for a proactive and enduring Carthaginian presence in the Iberian Peninsula as early as 348 BCE. A close examination of the treaty’s contents, Polybius’ comments on it, and the other evidence available – including two problematic entries in Stephanus of Byzantium’s Ethnika – reveals that Mastia Tarseion was in fact a promontory on the North African coast, west of but not very far from the city of Carthage. The treaty thus emerges as a key document for understanding the nature and evolution of Carthaginian imperialism in the western Mediterranean.