As the Tampa Bay Rays and Philadelphia Phillies prepare to slug it out in the World Series, a baseball contest of a different sort—involving some of the sport's tiniest players—is raging around America's suburban fields.

At the center of the tug-of-war: children's baseball uniforms.

On one side is Major League Baseball, which has been asserting its trademark rights and collecting licensing fees through an official uniform supplier, Majestic Athletic, Ltd. M.L.B. has threatened legal action against vendors who put pro team names on jerseys worn by Little League and amateur players without permission.

On the other side, some youth leagues are crying foul. They denounce the higher cost of official uniforms as well as what they feel are M.L.B.'s hardball tactics to force them to buy from a single supplier. So some are boycotting pro team names.

In Santa Clarita, California, a city north of Los Angeles, the William S. Hart league's baseball division narrowly voted to continue using names like the Mets and the Orioles. Its 1,800 players will still wear official hats bearing team logos, but the league plans to drop all pro names from team jerseys, to avoid M.L.B.-licensed apparel.

Others have kicked the Major League off the field altogether, switching to generic collegiate monikers like Trojans and Bruins, or looking to the rainbow. In Bloomingdale, Illinois, where miniature Pirates and Marlins once faced off, teams are now identified by color.

"At first the younger kids were disappointed...but you're saving the membership thousands of dollars," says Rick Palandri, a board member of Bloomingdale Baseball and Softball Association. The league's moisture-wicking jerseys—imperative, Palandri says, for keeping boys and girls cool in summer—cost about half the price of comparable M.L.B.-licensed outfits.

Out of more than 10,000 youth-baseball leagues across America, about 4,000 wear M.L.B.-licensed uniforms. No one knows how many use pro-team names but avoid their mention on jerseys, or copy on the sly—like one in California rumored to import knockoffs from Mexico.

To police sales, M.L.B. has been sending warning letters to small vendors and making them sign agreements prohibiting the use of its team names in any form, even without official logos, on the athletic apparel they sell.

"Major League Baseball is creating a monopoly for its products," says Fred Miller, vice president of Evans TeamWear, an Orange County company that provides uniforms to more than 100 youth leagues in California and Nevada.

Evans recently paid M.L.B. a $75,000 settlement over unpaid royalties after being contacted by M.L.B. lawyers—though Miller says his company did nothing wrong. "It comes down to a business decision," he said. "Do I want to fight this in court for [a lot more], or pay the settlement fee now?"

But pro-baseball officials say they must protect their trademarks as a matter of law. In 2005, M.L.B. entered into a licensing arrangement designating Majestic, an apparel manufacturer in Bangor, Pennsylvania, as the exclusive supplier of all uniforms bearing its team names—from pros in the Major Leagues to kids in Little League and casual fans everywhere.

"We're obligated by our contracts," said Matt Bourne, spokesman for M.L.B., the corporate entity representing the 30 pro-baseball clubs. "If you don't protect your intellectual property, there's a risk of losing ownership. Team names are our identity. It's critical to the brand names we've built for more than 100 years."

It's also big business. The Sporting Goods Manufacturers Association said the retail value of all Major League Baseball-licensed merchandise—including uniforms and memorabilia—totaled $3.3 billion last year, the highest for any professional sport.