From the Catholic Encyclopedia:
No mention of St. Bartholomew occurs in ecclesiastical literature before Eusebius, who mentions that Pantaenus, the master of Origen, while evangelizing India, was told that the Apostle had preached there before him and had given to his converts the Gospel of St. Matthew written in Hebrew, which was still treasured by the Church. "India" was a name covering a very wide area, including even Arabia Felix. Other traditions represent St. Bartholomew as preaching in Mesopotamia, Persia, Egypt, Armenia, Lycaonia, Phrygia, and on the shores of the Black Sea; one legend, it is interesting to note, identifies him with Nathaniel. The manner of his death, said to have occurred at Albanopolis in Armenia, is equally uncertain; according to some, he was beheaded, according to others, flayed alive and crucified, head downward, by order of Astyages, for having converted his brother, Polymius, King of Armenia. On account of this latter legend, he is often represented in art (e.g. in Michelangelo's Last Judgment) as flayed and holding in his hand his own skin.