(Εὐθύδημος), king of Bactria, was a native of Magnesia. (Plb. 11.34.) We know nothing of the circumstances attending his elevation to the sovereignty of Bactria, but he seems to have taken advantage of dissensions among the descendants of those who had first established the independenceof that country, and to have wrested the sovereign power either from Diodotus II. or some of his family. He then extended his power over the neighbouring provinces, so as to become the founder of the greatness of the Bactrian monarchy, though not the actual founder of the kingdom, as has been erroneously inferred from a passage in Strabo. (Strab. xi. p.515; Plb. 11.34; Wilson's Ariana, p. 220.) Antiochus the Great, after his expedition against Parthia in B. C. 212, proceeded to invade the territories of the Bactrian king. Euthydemus met him on the banks of the Arius, but was defeated and compelled to fall back upon Zariaspa, the capital of Bactria. (Plb. 10.49.) From hence he entered into negotiations with Antiochus, who appears to have despaired of effecting his subjugation by force, as he was readily induced to come to terms, by which he confirmed Euthydemus in the regal dignity, and gave one of his own daughters in marriage to his son Demetrius. In return for this, Euthydemus lent him his support in his Indian expedition. (Plb. 11.34.) The commencement of the reign of Euthydemus may be referred with mach probability to about B. C. 220. (Wilson's Ariana, p. 221.) Silver coins of this prince, of Greek style of workmanship and bearing Greek inscriptions, have been found in considerable numbers at Bokhara, Balkh, and other places within the limits of Bactria, thus attesting the exteat to which Greek civilization had been introduced into those remote regions. (Ibid. p. 222.)