The Jubilee Church's founder and pastor, Bishop Gilbert A. Thompson, expects 2,000 of its members who now travel to Mattapan from Stoughton, Avon, and Brockton to attend the new church, which will be called Jubilee South and is expected to open in the fall.

The expansion suggests that the worldwide megachurch movement is having an impact in the Boston area, where the Roman Catholic Church has long dominated the religious landscape.

''It's not rocket science," said Thompson, 59, who also leads Boston's Black Ministerial Alliance. ''If 2,000 are driving out of their own towns into Boston, gee . . . we need to plan another church. It's one of the innovative things that modern pastors are doing."

Thompson's church is already one of the largest in Boston.

The structure that housed Our Lady of the Rosary, built in 1958, will not be torn down, but Thompson said the music system will be upgraded and the pews will be replaced with chairs, to accommodate more people.

The purchase is in addition to other expansion moves Thompson said are underway. The church is opening an office building in Dudley Square, he said. Building a new campus is also a possible future move on 25 acres of land in Roslindale.