If he didn't value his privacy so, Neil Nichols could claim to be the taxi czar of the Washington area. And he probably wouldn't get a big argument about it.

Take a Red Top Cab -- Arlington's largest cab company. It belongs to Nichols.

Or call Red Top's rival, Arlington Yellow Cab Co. That's Nichols' company, too.

So is Air Transit Co., which has the exclusive franchise to provide cab service to Dulles Airport through its Airport Cab subsidiary.

And so is Airport Limo Co., which replaced Greyhound as operator of the bus-limousine service from downtown Washington to both National and Dulles airports.

In fact, it takes some effort to go anywhere in Northern Virginia by cab or limousine without calling a Neil Nichols company. He also is an owner of Yellow Cab companies serving Annandale, Baileys Crossroads, McLean, and Falls Church. (The other Yellow Cab companies in the Washington area operate independently.)

Add Nichols' All State Messenger and Delivery Service, which delivers packages around the area, and Murphy Brothers, a gasoline company that provides fuel for Nichols' transportation fleets, and you have the largest, most diversified transportation network in the Washington metropolitan area.

"Mr. Nichols is one of the premiere operators in the country," says Al LaGasse, head of the International Taxicab Association, a trade organization with offices in Gaithersburg. Nichols' innovations often are cited in taxicab trade publications, competitors note.

"He's got a large, progressive outfit. Very aggressive," adds LaGasse.

And yet despite the size and spread of his ventures, Nichols' name is unknown to his customers and the public generally, associates and competitors agree. "He's just not interested in publicity," says one former associate.

"He's not shy," the former associate said. "But he eats, sleeps and drinks the business," the former employe said. "I just don't know of anything he does outside the transportation business."

Nichols doesn't quarrel with that assessment. "The taxicab business is basically a Mom and Pop operation. You have to wind them up every morning or they don't run," he said in an interview. Winding them up takes most of his time. Beyond that, he "likes to keep a low profile," he says.

Nichols grew up with taxis. His father, Waldo Emerson Nichols, became president of Arlington Yellow Cab in 1947 and, under his supervision, it doubled in size from 25 cars to 50 by the early 1960s. Neil Nichols started out with his father, driving, repairing, and dispatching cabs, associates say.

But Waldo Nichols died of a heart attack in 1963, at age 57, and the family lost control of Arlington Yellow Cab to a business associate of his father, Neil Nichols said.

So he and a partner, Robert Nolting, formed a competitor they called Red Top Cab Co. in 1965 and took Yellow Cab, starting from scratch with only five taxicabs.At the time, Nichols was a minority owner in Yellow Cab, but that didn't stand in the way.

"We competed heavily," he said, and won. Red Top's growth was dramatic. The fleet of five cabs numbered 35 in a year and grew to 189 today. At the same time, Nichols was picking up other cab companies from owners who couldn't make it. By 1970, Arlington Yellow Cab was failing financially, Nichols said, and he bought back the old family company.

Today, more than 400 cabs operate for the various Nichols companies, he estimates. Red Top is the leader in Arlington and, with Arlington Yellow Cab's 100 cabs and the smaller Yellow Cab outfits, Nichols' fleet is easily the largest in the Washington area. His chief competitor, Diamond Cab of Alexandria, has 90 cabs in Arlington and 167 in Alexandria. Yellow Cab of Alexandria -- not one of Nichols' companies, has 159 cabs.

The size of Nichols' fleet is even more impressive than those numbers suggest, taxi licensing officials say. More than its main rivals, Red Top operates with company-owned cabs that are leased to drivers a week at a time. Other companies rely more on independent cab drivers who own their own cars and operate with a company's name and colors.

There was nothing new about Nichols' strategy of merging smaller cab companies into a large organization. But much earlier than most, he understood the need to diversify into other kinds of transportation services, said LaGasse. "That's the key to survival today," he said.

A company that owns cabs and vans can't make them pay solely by serving rush-hour customers, LaGasse said. That is why successful companies are providing other services -- delivering prepared meals to the homes of invalids, transporting handicapped and other students who received special educational services outside school. "It's not a taxicab service anymore; it's got to be a transportation system," LaGasse said.

The size of Nichols' transportation network proved to be a crucial factor in winning the Dulles contract, which gives his company exclusive right to pick up arriving passengers, according to a spokesman for the Federal Aviation Administration, which oversees transportation services at Dulles and National airports.

To win the Dulles contract, Nichols' Air Transit Co. had to promise to have up to 60 air-conditioned cabs available on 10 minutes notice between 6 a.m. and 11 p.m. Air Transit's winning bid also included a promise to pay the FAA $21 per thousand passengers arriving or departing Dulles by air, or $31,000, whichever was greater, in the contract's first year, 1975. The minimum payment has risen to $42,700, an FAA spokesman said.

The FAA, which will rebid the contract this year, gave Air Transit a six-month extension, an indication the agency is not dissatisfied with the company's work, a spokesman said.

The only recent public attention Nichols' companies have received has not been favorable, however.

On Feb. 29, the Environmental Protection Agency charged that during the gasoline shortages in the spring of 1979, some of Nichols' drivers were buying leaded gasoline from Nichols' fuel company, and putting it into their "unleaded only" taxis, a violation of EPA regulations. Leaded gasoline can wreck the expensive catalytic pollution control devices on autos. But during the gas shortage, there was more leaded for sale than unleaded, and the price was cheaper.

According to EPA's complaint, 22 drivers for three of Nichols' companies had illegally purchased the unleaded gasoline on 44 occasions from Murphy Brothers, Nichols' refueling company located in Falls Church. The drivers were not implicated because the pump was mislabeled "unleaded," an EPA spokesman said.

The EPA has asked for civil penalties of $271,000 from each of four Nichols companies -- Red Top, Arlington Yellow Cab, All State Messenger and Murphy Brothers -- putting the potential total penalty at more than $1 million.

A preliminary hearing on the case probably will be held later this year unless the EPA and Nichols' companies reach a settlement, an EPA official said.

The impact of such a financial penalty on Nichols' network isn't apparent because his interests are divided widely among separate corporations, none of which issue public financial reports.