The city’s drive to deny New Yorkers access to fireworks traditionally used to celebrate July 4 is hurting some Pennsylvania sellers, storeowners said.

“The actions of the NYPD are working,” the owner of Phantom Fireworks, William Weimer, said. “There are a fair number of customers who have chosen not to purchase.”

Fireworks vendors along the border with New York are in a peculiar jam. Although situated in Pennsylvania, they aren’t allowed to sell most types of fireworks to Pennsylvania residents unless they have a permit. And with the New York Police Department waiting to pounce on New Yorkers returning to the city with trunks full of illegal fireworks, the usual flow of customers to the outlets has thinned this year, storeowners said.

It is illegal to possess fireworks in New York. State police, however, aren’t actively seeking out illegal fireworks the way the NYPD is, a spokesman, Sergeant Kearn Swoboda, said. State troopers rely entirely on complaints and tips about illegal fireworks shipments, he said.

The city’s police department has taken the cause a notch further, going so far as to send detectives into Pennsylvania to conduct surveillance in the parking lots of fireworks stores. Storeowners said they’ve already wised up to the tactic.

“We’ve gotten pretty good at picking them out,” the manager of the Phantom Fireworks store at the Delaware Water Gap, Tom Allison, said.

Responding to a small hike in the number of illegal fireworks used last year, Mayor Bloomberg and the police commissioner, Raymond Kelly, have upped the stakes for offenders and ordered rigorous enforcement. A person bringing fireworks into the city can have their car seized and face hefty civil penalties, as well as criminal charges.

Between January 1 and June 26, the Joint Fireworks Task Force – made up of police, firefighters, and other public safety officers – arrested 109 people for possession of illegal fireworks. Less than 40 were arrested for the same period last year. Police have also seized 48 vehicles and 1,404 cases of illegal fireworks.

In early June, the mayor called on New Yorkers to report sightings of fireworks to 311, arguing that all fireworks – big or small – should be left to the professionals. If the report results in a seizure, the caller is eligible for a reward of up to $1,000.

So far this year there have been no reports of injuries caused by fireworks, according to police and fire officials. In 1995, the year before Mayor Giuliani began a campaign against fireworks, there were 56 fireworks-related injuries during the period during the usual window of time when such injuries happen, May 1 to July 4.

“Fireworks are a wonderful way to celebrate Independence Day, but it’s critical that we leave it to the professionals,” Mr. Bloomberg said recently. “Fireworks are not only illegal, they are dangerous and in the hands of an untrained individual, fireworks can have deadly consequences.”

It is no secret that Pennsylvania fireworks stores make most of their money from out-of-state buyers. Residents from as far north as Vermont and as far west as Ohio come to the state to get their fill of pyrotechnics.

One of the top-selling fireworks at Phantom Fireworks is the New York Salute Finale, a 500-gram “repeater” that lights up the sky with multi-colored, floral scintilla.

To capitalize on this demand, one seller, Keystone Fireworks Superstore, took out advertising space in the New York City edition of Clipper Magazine, giving directions to Pennsylvania from Port Jervis, N.Y. The company also has one billboard up on a major New York highway leading to the Poconos. Nearly all the fireworks stores that have Web sites give directions to the store from cities in New York.

With the crackdown in full force, Phantom Fireworks and others have put up fliers at the counter warning New Yorkers of the reports of surveillance and possible penalties. Many are returning home empty-handed, Mr. Weimer said.

“Fireworks are part of the American fiber,” he said. “It’s like American Pie and motherhood. I recognize that it’s terribly inappropriate to be shooting fireworks on the streets of Manhattan. There have to be reservations, but that doesn’t mean that fireworks can’t be used properly in the outer boroughs.”

The fireworks that are most dangerous are illegal across America – explosives like M80s, Silver Salutes, and Cherry Bombs, the executive director of the American Pyrotechnic Association, Julie Heckman, said.

“We can applaud that kind of crackdown,” she said. “Those are deadly explosives, they are federally banned. But it seems a little ridiculous to penalize somebody for using sparklers.”