The longest-running opera at Wolf Trap this summer is not "The Barber of Seville" or "L'Ormindo," which are allotted four performances each, and certainly not "A Midsummer Night's Dream," which will have only two. Richard Wargo's "The Music Shop," a funny, unpretentious, tuneful little confection that runs a bit less than an hour, is scheduled for five performances, through Aug. 8.

You may not have heard of it because it runs at 11 a.m. Saturdays (including today) at the Children's Theatre-in-the-Woods -- not the best known part of Wolf Trap. Tickets are free but you should phone (703) 255-1827 for reservations.

And you have probably not heard of Wargo because he is young and American -- one of those very clever composers of the post-Menotti generation who can write their own librettos as well as scores. If ability were always recognized and rewarded, and if money were no object, Wargo and other composers would be building a golden age of musical theater on this continent. As it is, he is providing prime entertainment for children at one of Wolf Trap's less well known facilities.

The production is stripped down but professional. "This silver thing down here -- you pretend it's an orchestra; it's really a tape recorder," music director Carol Palca tells the young audience, introducing the show. After a few minutes, one forgets that the accompaniment is mechanical because the four young men and women in the cast are anything but. Jeffrey Schneider's scenery and Gregory Bolton's costumes would fit well in any opera house. The lighting lacks subtlety, being provided entirely by the sun, but the stage direction of Nicholas Ivor Martin is imaginative, lively and full of broad, vivid gestures calculated to delight an audience of children.

"The Music Shop" is based on a short story by Chekhov and lightly, humorously tinged with his special flavor of desperation. Ivan, a customer in Dmitri's music shop, is married to a singer who will be performing at a prince's wedding. At the last minute, the prince has asked for a particular song; Ivan must buy the sheet music, but he doesn't remember the title. One of the opera's key numbers, "He Forgot to Write It Down," deals with his feelings of guilt.

But the most spectacular number comes when Dmitri and his assistant Masha try to jog Ivan's memory by singing every tune they have in stock: Mozart's "Alleluia," "The Volga Boatmen," the Seguedilla and "Toreador Song" from "Carmen," the slow movement of the "New World" Symphony, the "1812" Overture, the drinking song from "La Traviata," "Vesti la Giubba," the "Anvil Chorus," the "Largo al Factotum" and many more. Could a child have a more vivid or wide-ranging introduction to the delights of opera? Could an adult opera-lover find a funnier variation on "Name That Tune?"

The singers -- Ann Panagulias, Nicholas Solomon, Paul Austin Kelly and Margaret Jane Wray -- are members of the Wolf Trap Opera Company. They have performed or will perform this summer in other productions that have a higher level of visibility. But they are giving as much skill and dedication to "The Music Shop" as to the adult productions -- quite rightly, because nothing they do is more important than showing children the wonders of opera.