The City of Fairfax Band had just made its debut on the Rehobeth Beach ocean front last summer when the bus carrying band members home broke down a few miles from the Bay Bridge.

The group descended from the bus, instruments and all, and, in a cornfield beside Rte. 50, gave the backed-up bridge traffic an impromptu concert.

Indeed, the musical creativity and camaraderie of the 70-member volunteer band have given it high marks in an area that is loaded with orchestras and service bands.

The City of Fairfax Band attracts an audience of between 300 and 400 loyal listeners, some of whom come from the District.

Offering residents and the towns they represent an inexpensive respite from the daily business bog, community bands across the country have maintained their popularity over the years.

There are more than 2,000 community bands in the United States, including more than 20 volunteer bands in the Washington metropolitan area, that perform seasonal free concerts in parks, at monuments, in high school stadiums and in holiday parades.

Warren A. Wilson, president and business manager of the Allentown Band that, at 157 years, is the oldest civilian band in the country, said the groups serve a dual purpose for the small towns they represent.

"The band gives the musicians a place to play and it provides the citizens with good entertainment," Wilson said. "It's a part of Americana that should be preserved."

"We're kind of like the American Legion . . . always there and eveybody knows about it," said Daria Parrell, who, along with her husband, Richard, has been with the Fairfax band since it began in 1969.

At the outset, Parrell said, only a handful of residents would show up at the evening lawn concerts to watch the then 14-member band perform.

"We used to get 30 or 40 people there," she said. "But Fairfax City's acceptance of us grew along with the band. Now we have scientists, engineers, librarians and students in the group."

Band director Thomas H. Hill said that one-third of the band's members are students.

"A lot of returning students play with us after they graduate from college," he said. "There's also a sixth grader who comes with his father and they each play the baritone."

Hill stressed that the band mostly gives concerts, but also marches in the city's Fourth of July parade. And, in November 1981, the band performed with the French Marine Band and the German Army band as Virginia's representative in the 200th anniversary celebration of the Battle of Yorktown.

Bill Rosenmund, who met his wife, Joann, at band practice two years ago when they both played the same part on the French horn, said that an enthusiastic audience is the key to bringing a band to its peak performance.

"I can sit there and play by myself during a practice, but in a performance you can feel the audience's reaction. Music is something interfaced between the ensemble and the audience," said Rosenmund, a Rockville-based computer consultant. "When there's a lot of audience support, we play better."

The City of Fairfax Band is sponsored by the city's Department of Recreation and has received $5,000 from the city council for its 1985-86 season. The money will pay the band director's yearly salary of $2,400, the assistant director's salary of $500, and provide $450 for music sheets, $500 for promotion and supplies, and $150 to pay a band librarian to research and select compositions.

In addition, members pay $7 in annual dues toward the cost of special equipment and group trips.

Mayor George T. Snyder Jr. said he would like to see the city's business community take a front seat when it comes to financially supporting the coed group.

"To my knowledge, the council is giving the band what it needs, but I would like to see more private-sector support," Snyder said. "It's a great band and, if it can be 100 percent supported by the private sector, well, that's fine."

Snyder described the band's summer evening lawn concerts as a "little Wolf Trap . . . . but you don't have to buy a ticket. You just sit there and .elax."

The band will perform five free evening concerts this summer at 8 p.m. on the lawn in front of City Hall on Armstrong Street in Fairfax. Their repertoire includes the brassy sounds of John Philip Sousa, Broadway show tunes, sonotas and gospel.

The band gives 16 concerts during its summer season, which runs through Aug. 17. Then it takes a four-week break before the group starts its fall season in mid-September.

The band practices every Wednesday evening from 7:45 to 9:45 p.m. in Fairfax High School's band room. Only 20 band members live in Fairfax City; the others live in Springfield, Great Falls, Takoma Park and Manassas.

Trumpet player David R. (Ray) Abell, whose wife and daughter also are band members, said that the band relaxes him during his work week.

"The Washington, D.C., area is very hectic and fast-paced," Abell said. "The band is a change that is necessary . . . it gets your mind off business and gives you a chance to dream a little bit."