At Colgate Close Park in the South Bronx, a baseball diamond with grass so green it looks like a mirage in the otherwise gray, industrial neighborhood sits behind a locked fence. Teenagers from the neighborhood play catch on an adjoining patch of asphalt.

They cannot use the city field because it is reserved by the Parks and Recreation Department for players who belong to organized sports leagues, which typically charge membership fees.

Across the city, in fact, many of the best fields have been padlocked by the Parks Department.

While only 64 — or about 10 percent — of the city's total diamonds are kept locked, the Parks Department has increasingly turned to locking its fields as a solution to overuse and vandalism. Padlocked fields did not exist or barely existed when players like Manny Ramirez, Sandy Koufax, Willie Randolph and Joe Torre honed their skills on the city's public diamonds before becoming major league stars and winning the World Series as players — or in Mr. Torre's case, as a manager.

Because some of the city's best fields — which cost millions of dollars in public funds to build, including $1 million for the diamond at Colgate Close Park — are now essentially off limits to families who cannot afford to pay league dues, critics say a type of privatization of the limited number of good fields has occurred.

''Talk to any coach or player or athletic director and they all say exactly the same thing, that the fields are dangerous to play on, and for decades the city has refused to maintain them,'' said Geoffrey Croft, president of New York City Parks Advocates, ''Now they've taken to locking up the best fields.''

The parks commissioner, Adrian Benepe, acknowledged that the condition of many of the city's 631 baseball and softball diamonds was less than perfect, saying the problem of keeping the city's ball fields in good playing shape was vexing. However, he said the city's diamonds were in their best shape in 25 years.

Mr. Benepe said that because of a confluence of events — including a sharp rise in the number of adults who have taken up recreational sports in recent years — the number of permits to use the city's baseball and softball fields had doubled over the past six years, to 136,890 in the 2005 fiscal year. Given the intensive use, he said, baseball diamonds are almost impossible to keep in decent playing shape unless they are open only to organized groups who have permits.

''You add that all up and you put a tremendous strain on existing maintenance,'' he said. ''So grass fields definitely took a hit. Even if you have perfect Yankee Stadium-type maintenance, grass is not a practicable surface if you want unlimited play.''

He said that the Parks Department was in the midst of a $50 million effort to repair 55 ball fields, most of which are accessible to the general public, but some of which will most likely be kept padlocked once they are resurfaced.

Mr. Benepe said that for now, children and adults who play baseball and softball outside of organized leagues will have to make do with the city's unlocked diamonds — often dirt fields featuring modest patches of grass with infields that are strewn with glass and rocks and outfields that are filled with holes.

''If you want to play pickup, you're not going to have a perfect field,'' he said.

Still, he said he had never received a complaint from any person or group denied access to a city diamond during his four years as commissioner.

The Parks Department said it adopted the policy of locking fields to stave off deterioration, as the use of the fields rose sharply over the past several years and maintenance did not. The department does not keep separate budget figures for the cost of maintaining its unlocked fields, but officials said the diamonds did not have a staff dedicated to their upkeep. They are maintained along with the other areas of the parks they are in.

On the other hand, the department's locked fields, which are maintained by private groups, are usually watered, raked and mowed several times a week during baseball season.

''We lock them to prevent overuse, because unless you can limit the overuse in some way, you are not going to have a high-quality playing field,'' said the Parks Department's first deputy commissioner, Liam Kavanagh. ''The groups that work on these fields have to know their investment is going to be there when they come out there to play every spring.''

One example of the current arrangement is a privately maintained and gated Parks Department baseball diamond known as Co-op City Field, near Co-op City in the Bronx, a development that has about 50,000 residents, many of them children.

The well-tended field is operated by the Co-op City Little League as a virtual private enterprise. Several other ball fields in the area that had been open to the public have been paved over during the past several years to make room for additional parking. The only other public diamond near the complex is riddled with rocks and glass, and residents say that no one has tried to play baseball there in years.

John Colon, president of the board of directors of the Little League, which is largely responsible for caring for the gated field, said the diamond's maintenance was paid for with profits from the field's snack bar, which range from $800 to $1,000 or more a week. The Little League charges each child $135 for a 35-game season between April and June. More than a dozen neighborhood businesses serve as sponsors in exchange for placards on the diamond's fence.

Mr. Colon said Co-op City Field would be ruined if he did not keep it gated with two fences and a lock. The Parks Department does not offer much help with upkeep, he said.

''We're not about closing our gates to people, but we can't have outsiders come in and doing what they will, ruining all the hard work we put in,'' said Mr. Colon, who volunteers his time.

Children who cannot afford to join the Little League are not allowed to play there, Mr. Colon said. ''It takes a lot away from the kids who don't play in organized sports and don't have the opportunity to play here,'' he conceded, but he maintained that if he did not restrict access, the field would also attract older teenagers and would be littered with beer bottles and condoms. ''It's supposed to be for the kids, but you've got to lock it up at some point,'' he said.

The Parks Department has spent $85.4 million repairing and adding baseball and softball fields during the past five years, and plans to spend $50 million more during the next two years.

The Parks Department said that at a minimum, its workers come out once each spring to care for each of the city's ball fields. But many coaches say the Parks Department fails to do even that, simply dumping a pile of clay near baseball diamonds and counting on those who use the field to spread it themselves.

At Colgate Close Park, Aaron Barrett and Daniel Rivera, both 15, said it was not fair that they were not allowed to play on the $1 million ball field, the only diamond in the neighborhood.

''They build it all nice and they don't let us play there,'' said Aaron.

Daniel said, ''The only time we're allowed is when we sneak on it.''

Correction: January 6, 2006, Friday An article on Oct. 26 about baseball fields around New York City that are off limits to players who do not belong to one of the private leagues that maintain them quoted John Colon, president of the board of the Co-op City Little League, as saying that children who cannot afford to join the club cannot play on the field it uses. After the article was published, Little League Baseball and Softball, the parent organization, contacted The Times to say its rules prohibit a locally chartered club from turning away players for any reason, including inability to pay. This correction was delayed by an editing lapse.