''THE Fantasticks'' opened at the Sullivan Street Playhouse on May 3, 1960, and received mixed notices. It almost closed that weekend. Instead, the play is now celebrating its 25th-anniversary year. And on Sunday this Tom Jones and Harvey Schmidt version of a Rostand play will give its 10,000th performance - making it the longest-running show in American theater history.
The world record-holder, Agatha Christie's ''The Mousetrap,'' opened in London in 1952, but it changed theaters in 1974. ''The Fantasticks'' still occupies its original Greenwich Village home.
Such is the stuff of theatrical legends. And ''The Fantasticks,'' which has become a legend in its time, began in quite an unspectacular way.
The year was 1959. Jones and Schmidt had for several years been working on their version of Edmond Rostand's ''Les Romanesques.'' The story, a spoof of ''Romeo and Juliet,'' concerns two fathers who invent a feud in order to make their children fall in love.
The young collaborators envisioned a big Broadway show in ''a Rodgers and Hammerstein mold,'' the popular model in those days.
''I always imagined everybody on real horses on the stage of the Winter Garden,'' Mr. Schmidt recalled in a recent interview here.
''Eventually,'' added Mr. Jones, ''the whole project just collapsed. Our treatment was too heavy, too inflated, for the simple little Rostand piece. It seemed hopeless.''
Enter Word Baker, a fellow graduate of the University of Texas with Jones and Schmidt. Baker had a job directing three one-act plays at a summer theater run by actress Mildred Dunnock at Barnard College. If Jones and Schmidt could create a one-act musical version of the Rostand play in three weeks, Baker would put it on at Barnard. They did, and he did. The little French play to which they had been introduced in Texas by teacher-director B. Iden Payne became ''The Fantasticks.''
Of the several Off Broadway producers who then expressed interest, Jones and Schmidt chose Lore Noto, who had first encountered parts of the Jones script when Word Baker used it in an acting class. Mr. Noto became the show's producer, and its most ardent champion.
''The use of the language in the Boy's monologue,'' he said later, ''and in the Girl's monologue was so beautiful and so poetic . . . that I had to have confidence in anyone who could conceive a thing like that. And when I saw it at Barnard, I knew that Tom and Harvey had created something that I recognized in my own mind as a remarkable artistic statement.''
When conventional theater wisdom indicated that ''The Fantasticks'' should close after its uncertain reception, Mr. Noto instead came up with $2,500 he had been holding in reserve as part of the show's $16,500 capitalization. That would keep the musical running while it found an audience. Jones and Schmidt imagined, quite incorrectly, that their producer was an eccentric millionaire. In fact, he was simply a theater professional who, as an actor, played the Boy's Father in ''The Fantasticks'' for 14 years starting in 1970 - itself some kind of record. Nearly 200 actors and musicians have appeared in the production over the quarter century.
Shortly after the opening, Mr. Noto made another bold move. He suspended performances for a week and took the show to East Hampton, Long Island.
''That helped us,'' Mr. Jones said. ''There were a lot of important people there who saw it at the John Drew Theater.''
The producer also began licensing other productions that first year - something that usually doesn't happen until the end of a New York run. The novel move paid off: As of this writing, there have been 8,228 productions in more than 2,000 cities and towns in all 50 states and the District of Columbia - including stock, amateur, college, and high school productions.
An even bolder move occurred in 1964, with a ''Hallmark Hall of Fame'' TV production starring Bert Lahr, Stanley Holloway, and Ricardo Montalban.
''That might have killed us,'' says Mr. Jones. ''At that point we were beginning to sag. Instead, the TV show turned the thing around overnight.''
Indeed, the show's first uncertain summer provided Jones (then appearing in the cast under the name of ''Thomas Bruce'') with one of the most vivid memories of this 24-year history.
''There used to be three little girls who lived next door to the Sullivan Street Playhouse,'' he recalls. ''They were aged about 5, 7, and 9 when we started, and they made friends with the actors. When things got really rough, they'd come in and add to the audience. They must have seen 'The Fantasticks' dozens and dozens of times, and I was always glad to see them out there. Their mother would put them in white gloves and dress them up, and I'd think, 'Oh, thank you, thank you!' ''
The family has long since moved from the neighborhood. But Tom Jones, or maybe ''Thomas Bruce,'' still thinks of the three little girls - who could now have children of their own to applaud ''The Fantasticks.''
As for Jones and Schmidt, they went on to write such Broadway musicals as '' 110 in the Shade'' (based on N. Richard Naish's ''The Rainmaker'') and ''I Do! I Do!'' (based on Jan de Hartog's ''The Fourposter''). They have just completed ''Grover's Corners,'' a musical version of Thornton Wilder's ''Our Town,'' which will be presented on Broadway next season.
The first of many foreign productions of ''The Fantasticks'' took place in Mexico City. A former movie star came out of retirement to play the 16-year-old Luisa of the musical romance. Nobody seemed to mind. In the succeeding years, there have been 453 productions in 66 countries abroad, with Japan and Scandinavia heading the list. ''The Fantasticks'' has done well in the British Isles, although it was not a success in London.
But one of the oddest overseas ventures concerns a proposed Paris production that Jones and Schmidt considered a bit too fantastic.
''Everyone wore black leather jackets,'' Jones recalls. ''The abduction was all done with switch-blade knives - because of the success of 'West Side Story.' I went to a lovely luncheon in connection with the event. Everyone there spoke perfect English - except the translator, who didn't speak a word of English. That made me suspicious, so I asked for a copy and had it translated. And we turned it down. It was later done in Paris and got very good notices, but it was not a success there. So maybe we should have let them use switch-blades, after all.''
''The Fantasticks'' has in fact crossed language and national barriers with remarkable agility. It has been produced, among other places, in Canada and Germany; in Saudi Arabia, Turkey, Czechslovakia, and Israel; in Australia, New Zealand, and Africa. It played Kabul, Afghanistan (before the Soviet takeover), and Tehran, Iran (before the Ayatollah).
And the show's hit song, ''Try to Remember,'' has become an international standard, having been recorded by countless vocalists, orchestras, and instrumentalists. As the show's 25th year begins, its future is perhaps best left to that lovely Jones-Schmidt ballad:
''Try to remember, and if you remember,
''Follow, follow, follow, follow.''