Mygdonia (/mɪɡˈdoʊniə/; Greek: Μυγδονία / Μygdonia) was an ancient territory, part of Ancient Thrace, later conquered by Macedon, which comprised the plains around Therma (Thessalonica) together with the valleys of Klisali and Besikia, including the area of the Axios river mouth and extending as far east as Lake Bolbe. To the north it was joined by Crestonia. The Echeidorus, which flowed into the Thermaic Gulf near the marshes of the Axios, had its sources in Crestonia. The pass of Aulon or Arethusa was probably the boundary of Mygdonia towards Bisaltia. The maritime part of Mygdonia formed a district called Amphaxitis, a distinction which first occurs in Polybius, who divides all the great plain at the head of the Thermaic gulf into Amphaxitis and Bottiaea, and which is found three centur