(Πτολεμαῖος), king of EGYPT, was the eldest son of Ptolemy XI. Auletes. He is commonly said to have borne the surname of Dionysus, in imitation of his father, but there appears to be no authority for this assertion. By his father's will the sovereign power was left to himself and his sister Cleopatra jointly, and this arrangement was carried into effect without opposition, B. C. 51. Auletes had also referred the execution of his will to the Roman senate, and the latter accepted the office, confirmed its provisions and bestowed on Pompey the title of guardian of the young king (Caes. Civ. 3.108; Eutrop. vi 21). But the approach of the civil war prevented them from taking any active part, and the administration of affairs fell into the hands of an eunuch named Pothinus. It was not long before dissensions broke out between the latter and Cleopatra, which ended in the expulsion of the princess, after she had reigned in conjunction with her brother about three years, B. C. 48. Hereupon she took refuge in Syria, and assembled an army with which she invaded Egypt. The young king, accompanied by his guardians, met her at Pelusium, and it was while the two armies were here encamped opposite to one another, that Pompey landed in Egypt, to throw himself as a suppliant on the protection of Ptolemy; but was assassinated by the orders of Pothinus and Achillas before he could obtain an interview with the king himself. (Caes. Civ. 3.103, 104; D. C. 42.3, 4 ; Plut. Pomp. 77_79; Appian, App. BC 2.84, 85 ; Strab. xvii. p.797.) Shortly after, Caesar arrived in Egypt, and took upon himself to regulate the affairs of that kingdom, and settle the dispute between Ptolemy and his sister. But Cleopatra, who now hastened to return to Alexandria, soon obtained so powerful a hold over the conqueror by the influence of her personal attractions, that it was evident the latter would decide the controversy in her favour. Hereupon Pothinus determined to excite an insurrection against Caesar, and secretly summoned the army from Pelusium under Achillas. Caesar was taken by surprise, and had to maintain his ground with very inadequate forces in a part of the city where he was vehemently assailed both by the army and the populace. Ptolemy himself was at this time in the power of the conqueror, but after the contest had continued for some time, he obtained permission to repair to the camp of the insurgents, under pretence of exercising his authority to reduce them to submission ; instead of which he immediately put himself at their head. Caesar, however, still defied all their efforts; and, meanwhile, Mithridates of Pergamus had assembled an army in Syria, with which he advanced to the relief of the dictator. Ptolemy now turned his arms against this new enemy, and took up a strong position on the banks of the Nile to prevent Mithridates from crossing that river. Caesar himself, however, quickly arrived from Alexandria, landed near the mouth of the Nile, attacked and defeated the forces of the young king, and followed up his advantage by storming his camp. Ptolemy himself endeavoured to escape by the river, but was drowned in the attempt. His death occurred either before the close of B. C. 48, or early in the following year. (Caes. Civ. 3.106_112 ; Hirt. B. Alex. 1-31; D. C. 42.7-9, 34_43 ; Plut. Caes. 48, 49; Liv. Epit. cxii.; Appian, App. BC 2.89, 90; Porphyr. apud Euseb. Arm. p. 118.)