Auditions will be held July 1 at Earls Court Exhibition Center in London.
On July 8, producers of "Harry Potter and the Half-Blood Prince" will start their search for a teenage boy to play Tom Riddle, who grows up to be the evil dark wizard Lord Voldemort.
Applicants must have identification to prove they are from Britain and are between the ages of 15 and 18.
Last year, about 15,000 girls lined up at open casting calls to audition for the roles of Luna Lovegood and Cho Chang for "Harry Potter and the Order of the Phoenix," which will be released next month.
The roles went to Evanna Lynch, who plays Luna, and Katie Leung, who plays Cho.
Lynch said she was stunned when she heard she'd gotten the part.
"I hoped it, but I didn't go so far as to think it," she said. "I knew everything about Luna. She was my favorite. I explored her."
The films are based on the books by J.K. Rowling.
Daniel Radcliffe plays Harry, Rupert Grint plays Harry's best friend, Ron, and Watson plays Hermione Granger.