For the last six years, Thomas Pasatieri has been one of a kind in American music, so far as he knows. That's because since the age of 26 he's derived a steady income solely from the composition of operas, just as people like Verdi and Puccini and Wagner and Donizeti used to do in the old days.

There are few enough composers today who can subsist off their music at all. And to do so in the present-day opera world, with its prohibitive production costs and its general disinelination for the new, is risky indeed.

Part of Pasatieri's trick is to turn them out at the prodigious rate of his 18th and 19th-century predecessors. He's just 32 and has already written 14 operas. That's more than most composers ever write.

No. 10, andperhaps the work to cause the greatest stir, is a three-act setting of Chekhov's "The Seagull," which will be given its East Coast premiere by the Washington Opera this Wednesday.

The strong cast that premiered "The Seagull" in Houston in 1974 will be intact except for one singer. Obviously, so demanding a play must have good acting as well as good singing. The versatile Frank Corsaro will direct Evelyn Lear, Patricia Wells, Evelyn Petros, Richard Stilwell and John Reardon.

But if Pasatieri has been a success at getting his works performed, particularly on the growing regional opera circuit, Pasatieri made it clear in a recent interview that the magnitude of success that he seeks has eluded him. He feels that he has gotten more than his share of publicity and less than his share of serious attention where it counts the most.

"Believe me," he says with boyish plaintiveness, "nothing would make me happier than to get a performance from the Met or the New York City Opera, but there's just no sign of interest."

The City Opera's Julius Rudel was quoted several years as saying that Pasatieri's "talent is still unformed, unfulfilled. Things are going a little bit too quickly and it all comes out too-easily for him." Rudel had not at that time seen "The Seagull," which has drawn its share of critical praise.

Newsweek, for instance, saw the opera as evidence that "the young composer was born to write dramatic and vocal music," but added a caveat about "a disconcerting Hollywood Broadway penchant for the superficial."

His subsequent full-length work, "Ines de Castro," fared less well with the critics when it premiered two years ago in Baltimore.

A newer work, "Washington Square," got good notices in New York this winter.

Pasatieri takes his knocks with resignation: "To be honest with you, there has been too much publicity. I didn't seek it. It was just the oddity of becoming as opera composer that generated all those interviews. To come extent it backfired, generating antagonism and jealously. The result is that more people have read about me than have heard my music."

He cites a remark by another composer in a newspaper article that Pastaieri's music is "trash". "Now that really hurt, because to my knowledge he's never heard a note of my music. And that wouldn't have happened without the publicity. I write trash, he should hear my music before saying so."

To those who say that Pasatieri's idion is too conservative and electic - "a neo-Menotti melange" wrote one critic of his television opera, "The Trial of Mary Lincoln" - he counters, "because my music is often tonal I'm accused of not being serious. I get angry about this. They say Menotti (a mentor of Pasatieri) sounds like Puccini and I sound like Menotti and before long they'll probably be saying somebody else sounds like me."

His principal composition teachers are both of the conservative ilk, Vittorio Giannini and Vincent Persichetti. He studied at Julliard and received its first doctorate.

Pasatieri made his first public splash while studying under Darius Milhaud, at the 1964 Aspen festival where his chamber opera, "The Women," was produced and won the festival prize.

He has since composed a number of shorter operas, including the "Signor Deluso" performed by the Wolf Trap Company in 1974.

Five projects are pending, but Pasatieri will not enumerate them "because I'm superstitious about that." It was learned elsewhere that one is a short television opera for Beverly Sills. It is based on Eugene O'Neill's "Before Breakfast" and has been rewritten by Corsaro.

Pasatieri denies that economic necessity is a factor in the speed with which he turnes out operas: "Commissions run in cycles and about once every year and a half, I begin to wonder if they're going to run out but somehow they never do.

"And I write at the pace that comes naturally to me. i just have an irresistible impluse to play with notes. I enjoy the act of putting them on paper. I'll always do my best, And even if things go bad and I'm a total flop, I'll never deny myself the joy of writing music."