ANNAPOLIS -- Many local governments boast about their biggest industries and do everything possible to help them. But officials here in Anne Arundel County never discuss things with the biggest employer in the county. Never have, they say, and never expect to.

"They're very nice people," said Jeffrey Stone, the county's economic development director. "But they're not very cooperative."

The employer in question is the National Security Agency, the giant electronic spy of the federal government. Somewhere between 20,000 and 40,000 people work at its headquarters at Fort Meade next to the Baltimore-Washington Parkway, coding and decoding secret messages, eavesdropping on international telephone calls and picking up the casual radio banter of Soviet pilots. Most are civilians, and many are highly trained linguists and computer experts.

The heavily guarded complex of buildings, radio dishes, domes and towers, parking lots and fences is surrounded by signs warning people not to take photographs or even draw sketches.

Still more secret activity takes place outside the NSA complex in the growing number of office parks with unnamed offices nearby. Many of the high-technology firms will not reveal what they design or manufacture because it is classified. All this has created an unusually mysterious atmosphere in a small corner of normally friendly suburbia.

Also, the agency has provided a small economic boom. NSA is credited not only with hiring thousands of county residents but also with drawing in businesses and expanding the county's economic base.

What is happening in Anne Arundel is part of the growth regionwide, and particularly in Northern Virginia and Montgomery County, of businesses that do work for the Pentagon, the CIA and other government agencies with secret agendas.

The problem is, all the secrecy makes this prosperity hard to measure.

"We don't know how many suppliers they have or who they are," said Lissa Brown of the Anne Arundel County Office of Economic Development. "But we're guessing that the number is substantial and that they're probably the kind of companies that we are trying to attract to the county: high-tech, telecommunications, et cetera."

Many of these businesses around NSA are divisions of well-known firms: Motorola, Ford Aerospace and Telecommunications, and RCA Government Communications Systems, for instance. Others have technical-sounding names or names including the words "Data," "Signal" and "Science" that are not familiar.

NSA had about 7,000 contractors, whose business with the agency was worth about $900 million a year in 1977, according to James Banford's 1983 book on NSA, "The Puzzle Palace." But Brown, others in county government and developers in the NSA area say the number is certainly higher now.

Brown and other county officials point to several indicators suggesting the growing influence of NSA, which set up shop in Anne Arundel in 1957. The agency opened a large building in 1986 that county planners, viewing it from a distance, estimate has about 1 million square feet of office space and enough room for 7,000 employees. NSA, which already has office space near Baltimore-Washington International Airport, is leasing a large building in the newly completed Parkway Two office park on Dorsey Road.

County officials said they do not know how many people work there, and NSA will not tell them, insisting that the number is classified. The story is told within the county planning department that traffic engineers once put a rubber traffic-counting cord across a road near NSA and that armed guards came out and cut it.

Whatever the figure, NSA employs many more than the 16,000 people employed by Westinghouse Defense Electronic Group, which works largely with aircraft radar and is the county's second largest employer, according to Sandy Speer, the county government's demographer.

In addition, office parks continue to be built and high-technology firms are still moving into the NSA and BWI airport area. Developers, contractors and county officials who have watched the boom in high-technology firms say such construction has increased rapidly since 1982.

A large office park is being built by KMS Corp. developers across Baltimore-Washington Parkway from NSA. KMS officials refuse to discuss their dealings with NSA or its contractors, but county officials see the park as a clear sign that NSA wants more of its contractors close by.

NSA officials, of course, are not saying anything. NSA spokeswoman Cynthia Beck said information on employees and contracts is classified and cannot be divulged. Nor would she reveal what NSA's public relations staff does all day. However, she said, "We have plenty to do. We just don't answer the phone and say 'no comment.' "

NSA has a "substantial" impact on the county's economy, said J. Gilbert Haus, president of the Anne Arundel Trade Council and vice president of Dickinson-Heffner, a development and office leasing company that leases space to NSA and its contractors near BWI. Not only does NSA attract large numbers of profitable contractors, Haus said, but also its employees buy houses and spend paychecks here.

"I think their contribution is tremendous," said Stone, the county government's economic development director. "When you are faced with a recession or a downturn in government spending, the likelihood of NSA's budget being cut is probably pretty remote. So I think they're a very positive, long-range influence."

The effects of NSA are also felt in neighboring Howard County, where high-technology firms have clustered around Columbia. "NSA and Fort Meade have been a big factor in Columbia's growth," said Ed Ely, a vice president of the Rouse Co., which developed the planned town. "It's certainly been a major customer and a major supporter of development. It's clearly a motive for a great many companies to locate in Columbia."

But Ely, among others, said he cannot guess how many companies this means. "Some are clearly directly related to NSA," he said. "But the ones that are most directly related to NSA won't say what they do. A lot of their buildings have heavily secured areas that they simply won't let you in."

This protectiveness is felt directly by those working in construction trades in the NSA area. Steve Smith, president of a large air-conditioning and heating firm in nearby Columbia that does much work in "secure areas" of high-technology companies, said ducts and vents are often viewed by anonymous government inspectors taking photographs while workers are constantly accompanied by government escorts.

There are never enough escorts, Smith said. When a dozen men were working on the roof of a new building and one of them needed to use a bathroom, they all had to climb down and go together with the escort, Smith said.

Smith said he learned long ago not to ask too many questions. He once asked about the power source for a laser device in one of these buildings, he said, and was told: "You have no right to ask that question." He said people connected with these businesses refer to NSA as " 'our customer,' and you don't ask who the customer is."

Ronald Davis was working for an architectural planning firm in the 1950s when he was given a site plan project to work on. But the maps had no road names. Davis drew in new roads, parking areas and buildings. Years later, he said, "I drove by NSA and recognized it. I helped design the buildings."

Now a planner for Anne Arundel County, Davis said he attended a traffic meeting at which consultants working for Fort Meade passed out the seemingly innocuous results of a traffic study. A few days later, he said, Fort Meade officials went around collecting copies of the study, saying they were classified and should never have been given out.

But this sort of activity no longer surprises people here. Al Shehab, a retired Army colonel who lives in the nearby neighborhood of Odenton, said nobody raises eyebrows when a neighbor at a back yard barbecue refuses to discuss his work.

"It's second nature to us, and we don't give it a thought," he said. "We never ask them about what they do because we know it's classified and they're not going to talk about it. You know, there are plenty of other interesting things to talk about."