Coach Thomas Rongen chose 21 players for the tournament, which runs through July 22. There are 14 professional players, the most for a U.S. Under-20 World Cup squad, led by three-time U-20 World Cup participant Adu of Real Salt Lake.

Red Bull New York forward Altidore is the youngest player on the team at 17.

"We looked at about 80 players and narrowed it down to 21," Rongen said. "We feel that through playing games in qualifying and through camps, that we have a good handle on this age group and that we've made some great selections for this team to go forward and hopefully do something special during the World Cup."

The 18-year-old Adu played under Rongen in the U-20 World Cup in the United Arab Emirates in 2003 at age 14, and under Sigi Schmid in the Netherlands in 2005.

Adu and Michael Bradley are the only players on the roster to have played for the senior national team. Bradley has appeared in five games so far in 2007 with the senior squad, including appearances in the CONCACAF Gold Cup now in progress.

The Columbus Crew have Tim Ward, Robbie Rogers and Danny Szetela on the squad, the most by any club.

The United States opens against South Korea on June 30 at Montreal, where it also faces Poland on July 3. The Americans end group play on July 6 against four-time U-20 world champions Brazil at Ottawa.

The roster:

Goalkeepers: Brian Perk; Steve Sandbo; Chris Seitz.

Defenders: Amaechi Igwe; Ofori Sarkodie; Nathan Sturgis; Julian Valentin; Anthony Wallace; Tim Ward.

Midfielders: Freddy Adu; Bryan Arguez; Tony Beltran; Michael Bradley; Dax McCarty; Danny Szetela.