SOUTH BEND - William F. Roemer Jr., a South Bend native who spent 30 years chasing the mob as an agent for the FBI, died of natural causes in his home in Tucson. He was 69.

Mr. Roemer wrote two books about his FBI career and his efforts to combat organized crime.

Mr. Roemer retired from the FBI in 1980, but he never quit fighting organized crime. Mr. Roemer, who earned a law degree from the University of Notre Dame, became a lawyer representing people and firms sued by the mob. As a child, Mr. Roemer and his brothers often listened to radio shows like ''Gangbusters'' and imagined themselves bringing mobsters to justice.

Two of Mr. Roemer's brothers, Chuck and Tom, did go into law, and one of his brothers, Jim, is an administrator at Notre Dame. William Roemer is also the uncle of 3rd District Congressman Tim Roemer, D-South Bend.

He was born June 16, 1926. He married Jeanne Roemer. She survives with two sons, Bill of Tucson and Bob of Scottsdale, Ariz.; and four grandsons.

Memorial services will be at the University of Notre Dame. Cremation has taken place. Wolfrey Peace Chapel, Tucson, is in charge of arrangements.