The Battle of Cos was fought in ca. 261 BC or as late as 255 BC between an Antigonid fleet and a Ptolemaic fleet. Antigonus II Gonatas led his forces to victory, possibly over Patroclus, admiral of Ptolemy II. It has been widely assumed that the battle severely damaged Ptolemaic control of the Aegean, but this has been contested After the battle, Antigonus dedicated his flagship to Apollo.
The date of the battle is uncertain, although it must fall within the period 262-256 BC. Hammond dates it as late as 255 BC, but it is now increasingly placed in 261 BC.
^Janice J. Gabbert, Antigonus II Gonatas. A Political Biography. London and New York: Routledge, 1997. ISBN 0-415-01899-4. p.52
^Nicholas Geoffrey Lemprière Hammond, Frank William Walbank. A History of Macedonia: 336-167 B.C. Volume 3 of A History of Macedonia, Nicholas Geoffrey Lemprière Hammond Oxford University Press, 1988. ISBN 0-19-814815-1. Appendix IV