It was built for skateboarders after a grassroots effort that lasted years, but less than three months after the Martha's Vineyard Skatepark opened in Oak Bluffs, the skaters are dealing with some uninvited guests - bicyclists.

With half the wheels but 10 times as heavy, bicycles have rolled onto the scene at the skatepark, winning the right to share the park.

Peaceful coexistence is now the goal, but some skateboard backers are angry to see bikes encroaching on a facility that skaters - not bikers - worked so hard to build. They're also concerned about safety issues and damage to the park from bikes.

"We're not against bikers, but they should have their own park," said Rich Hammond, treasurer of the skatepark association. "This is not the place that was built for [bikes]."

Last month, Oak Bluffs parks commissioners disagreed.

Under pressure from bikers who hired an attorney and lobbied for access to the new ramps and half-pipes, the parks commission voted to end the ban on bikes at the skatepark and ordered the removal of a sign stating "No Bikes."

"We couldn't limit the skatepark to just skaters," said parks commission chairman Richard Combra Jr.

Edgartown attorney Candace Nichols, who represented one of the teenage bikers, convinced the parks commissioners that the bike ban violated a state recreational statute and increased the town's exposure to liability issues.

In essence, Ms. Nichols argued that because skatepark association leaders had been on-site to prevent bikers from using the facility, they were acting as supervisors of the park.

"The nonprofit corporation should refrain from oversight and enforcement of bikes at the park so as to avoid continuing to expose the town to liability," Ms. Nichols wrote in a Feb. 5 letter to parks commissioners.

Both sides of the bike and skateboard debate cited skateparks elsewhere in the state that either ban bikes or allow them access.

Mr. Hammond, in a letter to the editor in today's Gazette, lists nine towns in the commonwealth whose skateparks have kept bikers at bay. But Ms. Nichols and her bicycling clients also did their homework, producing letters from six town officials in Massachusetts that described mixed use at their skateparks.

"BMX bikers, skateboarders and inline skaters use the park together, and they all seem to get along," wrote Kathleen Crowley, recreation director in the town of Amesbury, in a letter dated Jan. 29. "The park has held up very well, with normal maintenance for wear and tear."

Back at the Island skatepark, the warmth of Monday afternoon beckoned skateboarders and bikers. Despite the political tension that preceded the bikers' access, there were no visible signs of rancor.

Four bikers pedaled amid the 20 skateboarders, nearly all of them teenagers or younger.

To Nate Sprague, an avid skateboarder, the advent of bikes in the skatepark isn't viewed as an invasion of outsiders. These bikers are his peers, kids he knows from school.

"It's a positive development," said the high school junior. "I'm trying to keep an open mind. It's the Island youth that matters, not who comes in and who doesn't."

Jon York, a biker and a high school sophomore from Edgartown, said, "We're all pretty much friends, just sharing."

As they shared the ramps, there was also room for praising stunts, no matter the number of axles. Mr. Sprague cheered on Ben Jones, who came off a ramp and took his bike in the air.

"Kids are pretty chill with it," said Mr. Jones, who is 16.

But back at the entrance, Mr. Hammond was watching the whole scene and not feeling quite so charitable. "Plastic wheels don't give the same wear and tear as pedals and handlebars on a bike when they go over," he said.

Summertime will be the real test. "The problem's in July when you've got 50 skaters in here," he added. "In summer, skaters will be restricted in their own park."

Clearly, some of the resentment felt by skateboarders is rooted in the fact that they shouldered the project for years, raising money at bake sales, lobbying the regional high school committee for use of the land and convincing voters in five towns to raise $20,000 for the project.

"I've been doing this personally for seven years," said Elaine Barse, president of the skatepark association and owner of a Vineyard Haven store that caters to skateboarders and surfers.

Ms. Barse said that a group of bikers did approach her board four years ago about being part of the project, but they lacked a solid plan for inclusion.

"They never came back and spoke with us until nearly the day the park opened," she said. "If we had been approached with a group as well organized as we were, we might have built the park differently."

It cost just over $100,000 to build the skatepark. With fund-raising, donated labor and money from Island towns, the skatepark association has no debts.

Ms. Barse said the two uses just might not be compatible for one park. "Bikes are heavier," she said, "and can cause more damage."

But Mr. Jones said that when he and other bikers talked to the skatepark association, the reception was frosty. "We went to the board, and they really didn't want to work with us," he said.

Now, with the ruling from the Oak Bluffs parks commission, the two groups are trying to strike a conciliatory note.

Greater participation from the public will help, said Ms. Barse. "I want to have a public meeting in April and get more people involved on the board," she said.

Money will continue to be an issue, and she is hoping that both bikers and skateboarders will share the burden of meeting maintenance costs and even raising money for more ramps.

Jeanne Staples, the mother of Ben Jones, is already feeling more of a vested interest in the skatepark. "He loves the activity," she said of her son. "To have a dedicated place where he and anyone can go is just wonderful. It's wonderful that it's here."