Searching for flights to Boston last summer, Juan Walte of Alexandria spotted a newspaper ad for Independence Air, a low-cost airline just starting service at Dulles International Airport.

He got a round-trip ticket to Logan International for $120, nearly half the price he normally paid. Now Walte, 65, and his wife, Ana, regularly fly Independence Air to visit family and friends in Boston and in southern Florida.

For frugal airline passengers such as Walte, fares are at their lowest in the Washington area in at least 15 years.

"I think this is a great market for somebody who bothers to shop around for prices," Walte said. "It's a buyer's market."

Driven by a high concentration of low-cost, low-fare carriers in the region, one-way fares in and out of Washington have fallen 39.5 percent since 1990, according to a study by Back Aviation Solutions, an airline-consulting group. The decline was among the steepest of any major U.S. city.

Washingtonians have benefited from having three major airports in the region -- Reagan National, Dulles International and Baltimore-Washington International -- each well served by low-fare airlines. Dulles is dominated by JetBlue Airways and also has operations of Independence and AirTran Airways, Southwest Airlines is the No. 1 airline at BWI, and AirTran and Spirit Airlines fly into National. On average, these carriers sell tickets at 40 to 60 percent discounts.

The lower prices do not stop with them. Larger airlines such as United Airlines, US Airways and Delta Air Lines have had to match budget carriers' fares to remain competitive. The steep discounts have added to the financial straits of the older legacy airlines because low ticket prices make it harder to cover their high costs.

The proliferation of budget carriers across the country has helped push down one-way fares outside Washington, too. Since 1990, ticket prices slid 34.5 percent in Los Angeles, 34.4 percent in New York, 32.7 percent in Boston and 22.7 percent in St. Louis. Fares have fallen much less in cities that have not had a large influx of low-cost carriers. Charlotte, for example, which is a hub for US Airways, has had ticket prices decline 18.2 percent since 1990.

On average across the country, fares are down 19.1 percent over the period, according to the study.

But even though fares have ratcheted down, finding the cheap seats sometimes can be maddening. Web site designer Joyce Brewer of the District has spent more than a month surfing the Internet in hopes of booking two seats to Miami over the Fourth of July weekend for less than $300.

"I know I paid less than this last year. Lower fares? Hogwash," Brewer said.

Fare experts point out that landing a cheap seat over a holiday weekend is as likely as finding a hot meal on your flight. What's more, do not expect to easily book discount seats from Washington on a Thursday evening or Friday afternoon with a return on a Sunday evening or Monday morning. Airlines know they can charge higher fares for those flights.

Booking further out for the summer also may pose hurdles, as many travelers have already grabbed many of the cheap seats. In addition, some airlines allot only a certain number of seats at the discounted price.

Heather Dolstra, vice president of Democracy Travel in the District, said she is already booking flights for clients traveling in August. More people are expected to fly this summer than in past years -- not only making reservations harder but also causing considerable congestion in the air and at the airports. About 200 million people are expected to board flights, up 4.1 percent from last summer, according to the Air Transport Association.

"We're seeing people book really far out. We're seeing day after day where flights are sold out, not just the discount seats," Dolstra said.

To help find the lower fares, Dolstra said, travelers should be more flexible. "If you don't like the fares at one airport, check the fares at another. Remember, this is high season."

Ticket prices in the Washington region have fallen so much partly because they started from a high base. The region had among the highest fares for a long time. With United dominant for decades at Dulles and US Airways having a lock on airfares at National and BWI, ticket prices did not fall much until Southwest began flying out of BWI in 1993.

Industry experts aren't sure how long fares will remain this low. Last month, many airlines raised round-trip ticket prices $10 to $20, the seventh increase since February. The airlines are trying to offset losses caused by high fuel prices and the run of low prices.

The future of Independence Air also will help determine whether fares continue creeping higher. The Dulles-based airline is struggling financially, and some analysts predict it could be forced to file for bankruptcy protection. The airline's parent, Flyi Inc., reported a loss of $103.7 million in its first quarter, much steeper than analysts had estimated.

It is also too soon to tell what impact a US Airways Group Inc. and America West Holdings Corp. merger would have on fares in the region. America West executives maintain that the combined airline -- which would operate under the US Airways name -- would have lower fares because it would adopt America West's budget pricing structure.

United Airlines, which has operated under bankruptcy court protection for 21/2 years, may cut flights or shift passengers onto smaller 70-seat regional planes to trim costs. If the airline uses more smaller planes, it may have to raise fares to offset the loss in available seats per plane.

Delta Air Lines also is struggling financially and may be forced to file for bankruptcy reorganization in coming months if it cannot secure help in funding a $450 million pension bill due this year.

And even Southwest is feeling the competitive pressure brought on by the fare battle. Gary C. Kelly, Southwest Airlines Co.'s chief executive, said Independence Air has begun stealing some of Southwest's passengers. As a result, Southwest's traffic at BWI has slid a little from last year's level.

"It's a real dogfight," he said.