The Diocese of Sault Ste. Marie was formerly established on September 16, 1904. Its territory extended west from Callander, Ontario for 800 miles, almost reaching the Manitoba border. On August 26, 1952, the western end of the diocese was separated to form the Diocese of Port Arthur, now known as the Diocese of Thunder Bay. The Diocese now comprises the Districts of Sudbury and Algoma, including portions of Nipissing District north and west of Lake Nipissing, and Manitoulin Island.