When a drug dealer is in trouble, he sometimes dials 911. But he isn't trying to reach the police.

Instead, this message is sent to a drug courier wearing a beeper that displays messages dialed from a phone: 911 means the police are closing in.

Although paging devices, or beepers, have grown in popularity throughout the labor force -- doctors, delivery people and journalists often use them -- they also have become a staple in the drug business, posing fresh problems for law enforcement and threatening to tarnish the image of a booming high-tech industry.

Sales of paging devices in the Washington area and across the nation jumped 25 percent last year, according to industry officials. In fact, the use of beepers, which confer status among young people, has become so prevalent that the Prince George's County and Baltimore school systems prohibit students from wearing them at school.

The increased use of beepers as part of the drug trade has caused Washington area beeper services to devise methods for weeding out illegitimate clients, such as tightening credit checks for potential buyers. Similarly, law enforcement officials have begun to refine tactics for combating the use of beepers in drug transactions -- including monitoring beepers and issuing subpoenas for devices seized in drug arrests.

Under Federal Communications Commission regulations, paging firms cannot refuse service to those they suspect of using their product for illegal activities -- a frustration for some sales people who said they usually can tell when a customer is involved in the drug trade.

"It's tough. Are we supposed to ask, 'Do you have priors {arrests}, sir?' " said Bill Evans, general manager of Mobilecomm.

The chief impact on local firms, according to sales people, is economic: Customers involved in the drug trade rarely pay their service bills, return their rented pagers or become long-term clients.

"We just don't want this business," said Rex Atwood, Washington area manager of Cellufone, a national high-tech communication company. "We've all had the experience of knowing we're not going to see the pager again."

Atwood said that in the past year, "the phone started ringing off the hook on Fridays" with people requesting pagers for the weekend -- a tipoff, he said, that they might be involved in the drug business.

Another clue, he said, is that drug traffickers prefer paging devices that indicate messages through vibrations instead of beeps. "It keeps them inconspicuous," Atwood said. Police officials said they have developed a profile of a beeper-using drug trafficker but will not reveal it.

About 6.5 million beepers are in use in the country, according to officials, although it is difficult to estimate what percentage is used for drug trafficking. U.S. Drug Enforcement Administration officials said that beepers, which have been used by bookies and cigarette smugglers, were introduced in the drug market about five years ago by Colombian cocaine organizations. Now, federal narcotics agents estimate that at least 90 percent of drug dealers use them.

Inspector Nelson Grillo, head of the District police morals division, said drug dealers elude police by communicating through beepers with young couriers who exchange the drugs and money involved in transactions. In this way, the dealer limits handling of drugs and risking stiff penalties.

Usually, one person telephones a number assigned to a beeper and a machine or a person relays the message to the beeper, which beeps, vibrates, triggers a voice-activated message or displays a code.

Special Agent Maurice Hill of the DEA said the use of beepers has become increasingly sophisticated; drug dealers use codes with as many as 24 digits to pass messages much more complex than the warning 911.

"A particular buyer will have an account number which he will add to the end of his {phone} number," Hill said. Or, he said, the numeral one "at the end of a number will mean the cocaine's in, {and} a zero will mean the cocaine's not in." More elaborate organizations develop more complex coding systems to transmit messages such as "the cocaine's been dropped at the usual place," Hill said.

For this reason, dealers are thought to prefer pagers that not only beep when there is a message but also have the capacity to display a number message, which is usually a phone number. These digital display beepers, which became available about five years ago, superseded tone-only beepers that require that the wearer call an answering service to get a message, industry members said. The most sophisticated beeper, which can transmit short voice messages, is not commonly used, according to local retailers.

A customer must be 18 years old to obtain a beeper because the service requires signing a contract. Several firm representatives say they have begun to double-check identification of current customers to ensure that they are of age.

Beeper service can be obtained through rentals or by buying a paging device that costs between $150 and $300 and can be hooked up to a paging service. The monthly service fee is $15 for those who own beepers; beeper rentals cost about $30 monthly.

A beeper is similar to a radio in that it receives transmissions from a central clearinghouse for messages. Each beeper has a unique crystal, known as a cap code, that allows it to receive its own messages.

To weed out bad customers, paging firms in the Washington area have begun instituting strict credit checks before providing service, limiting the number of beepers for which an individual can obtain service and setting fees for special services such as extra calls and number changes.

"We charge 25 cents per overcall, so the bills mount up more quickly and the service is shut off sooner," said Shawn Brashears, manager of marketing for Metromedia, a national company with the second largest market share in the region. She said the company has begun to charge $15 for number changes, a practice that drug dealers are believed to use to avoid tracking by the police. At the same time, she acknowledged that most dealers can afford to pay for the number changes, but she said that the increase in these requests had become an annoying drain on the industry's business.

On the local level, most companies began six months ago to tighten credit restrictions to prevent drug dealers from getting the service. If a client is willing to pay for a beeper in cash, a credit check is used to determine how many months of service the customer must pay for initially.

Some local firms may follow the example of businesses in Miami, which send out welcoming post cards to all new customers. If the cards come back "address unknown," the service is cut off, said Mike Frawley of Miami's Gold Coast paging company.

And after robberies in Miami in which $1 million in paging equipment was stolen in three months last winter, Metro Call will no longer service beepers unless a client can show a proper bill of sale, said Harry Brock, the company's president. The retail industry is working with the manufacturers to make it more difficult to remove serial numbers from beepers -- or to make beepers inoperable if the serial numbers are removed. Dealers, particularly in Miami, often take serial numbers off stolen beepers.

But members of the paging industry admit that such expenses will do little to prevent wealthy drug dealers from using beepers. At the same time, industry officials are concerned about driving up costs for legitimate customers. "I've seen people plunk down enough cash to pay for a whole year's worth of service," said Robert Edwards, chairman of CellCorp, a New Jersey-based firm. "They could buy the whole company, I think."

The only solution, industry members say, is to prevent dealers from buying beepers in the first place.

Last month, the industry trade association began a project to prepare a report for members on when service can be denied to customers.

The FCC, meanwhile, is not considering changing its rules to allow these firms to be more selective about clients for beeper sales and service, officials there said. Nor has there been much interest in getting them to do so.

"These firms should not be in the position of deciding what kind of messages are sent over pagers. That's not their responsibility," said Jerry Berman of the American Civil Liberties Union's legislative office.

While the industry grapples with the increasing demand of beepers for the drug trade, law enforcement officials also are trying to curb use of the devices for illegal purposes.

One way is to subpoena records of beepers recovered in arrests to discover who purchased or rented the devices. Paul Knight, assistant U.S. attorney for the District, says such records can aid in the development of conspiracy cases against drug ring leaders who rely on couriers to handle drugs.

Owners of paging services, here and in other urban areas, say that in the past year they have seen an increase in the number of subpoenas they receive. "It kept us busy. Law enforcement officers kept on seizing pagers, and we were in and out of court providing evidence on them," said Edwards, whose company once owned a large paging service in the New York City area.

Said Brashears of Metromedia: "The same person doesn't handle subpoenas too often for fear that he might be in danger."

Law enforcement officials with a court order can monitor phone messages to build a trail to and from a suspected drug dealer. An even more complicated practice is to build a clone of a beeper -- in other words, one that receives the same messages as an existing beeper -- to aid police and federal agents who are tracking drug dealers, said Fred Hess, chief of the office of enforcement operations in the criminal division of the Justice Department. The volume of requests for court orders to build clone beepers has increased 25 percent in the past year, Hess said.