Evidence for the Ptolemaic occupation of the Cycladic island of Keos in the 3rd century B.C. is both contentious and exiguous. A recent archaeological surface survey of both the territory and the polis-centre of Koressos (Ptolemaic Arsinoe, apparently the principal Egyptian foothold on the island) invites a review of relevant historical and epigraphical evidence, old and new. Contrary to some opinion, all the available evidence implies that the later Hellenistic period was not a time of material prosperity for Koressos. It is suggested that its incorporation into the Ptolemaic empire may have contributed to its demise as a city-state, and to its eventual transformation into a minor adjunct of the neighbouring polis of Ioulis.