Armstrong will be participating in the Masters South Central Zone Swimming Championships, the Austin (Texas) American-Statesman reported, and will be swimming against other middle-aged men.
U.S. Masters swimming doesn't drug test competitors and isn't under the jurisdiction of the U.S. Anti Doping Agency, which banned Armstrong from competing, the American-Statesman said.
Rob Butcher, the executive director of masters swimming, told the newspaper that the agency has no issues with Armstrong competing.
"The purpose of our organization is to encourage adults to swim," Butcher said.
Armstrong, 41, has entered the meet's three longest races -- the 500-, 1,000- and 1,650-yard freestyle. He's seeded second in the 1,000 freestyle and third in the other two events.
Armstrong, who began his competitive career as a swimmer, couldn't be reached for comment Wednesday.
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