Once you get to know it, there is no opera easier to fall foolishly in love with than Mozart's "Cosi Fan Tutte." So do not expect this to be an unbiased report. After all, "Cosi " is all about how people fall foolishly in love.

The Wolf Trap Company performed it last night, and will again tonight and Sunday.

Unlike the opera, the production is not perfect. It is merely good. But that tends to be the way with "Cosi " productions. Even the brilliant one at the Kennedy Center under the late Karl Bo hm during the Bicentennial had one or two casting weaknesses.

The Wolf Trap production has different kinds of strengths--two in particular.

One is the way the six principals in this ensemble opera sing together. That may sound trifling, but "Cosi " is one of those works that is more duets, trios including the very famous one that was featured in "Sunday, Bloody Sunday" , quartets, and other combinations. It's a very conversational opera. Arias are, for once, the exception rather than the rule, which is one reason for the sense of concentrated dramatic unity in this delicious comedy. It is tighter, for instance, than "The Marriage of Figaro," which, in its turn, is more diverse.

This kind of united ensemble comes only from practice, and the fact that these singers are working together every day on their nine-week residency showed throughout the evening.

The other special strength came from the ambiance, both acoustical and physical, of The Barns at Wolf Trap. The hall seats only about 350 people, and its warmth makes it an almost ideal place in which to stage an intimate opera. It is almost the obverse of the atmospherics of the old Filene Center, in which Mozart's opera sounded a little lost. The Kennedy Center's Terrace Theater, which is a bit larger, is cold and severe by comparison. It may sound extravagant, but one kept thinking of the theater at Glyndebourne in Sussex, which is where the modern revival of "Cosi " got under way in the '30s.

There is much to be said for singing "Cosi " in English, which is what the Wolf Trap company does, at least when the production is one that does not put a premium on vocal opulence. One reason that "Cosi " is the least famed of the four jewels in Mozart's operatic crown is that it is a little strange on first impression. The passionate revels of Don Giovanni and the revolutionary antics of Figaro somehow seem more accessible.

Here we have all this busyness about these virile Neopolitan lovers setting up this ruse to trap their betrothed. It all sounds pretty brainless until you begin to grasp the double and triple meanings of the superb text.

As for last night's soloists, who will do the repeat on Sunday, no one was brilliant, but neither was any one weak. And, given the odds, it was fortunate that the finest singing came from Barbara Hocher as Fiordiligi. If you're going to cast one role strongly, that's the one. Hocher showed fine Mozart phrasing, really excellent coloratura and splendid vocal heft in places such as the finale of act one, where Fiordiligi's voice must soar about the others in some of the most resplendent music Mozart ever wrote. Only her "Come scoglio" in the first act, the opera's grandest aria, seemed a little weak. The long lament in the second act was satisfying indeed.

The two men were also quite good. Of the two, Ferrando has the grander aria, and Bruce Ford sang it beautifully. Guglielmo had less to do, but he did it well.

Dorabella, the other sister, always seems to have to take a back seat vocally to Fiordiligi, which is really the story of her life in the plot as well. Lynn Beckstrom made the best of her plight.

Don Alfonso, who is really responsible for this mess that Mozart made so sublime, was wry and funny. And, of course, he did have his sublime moment in that famous first act trio.

Virginia Boomer was pretty funny as Despina in all her antics.

The orchestra and conductor Warren George Wilson were not the production's strong point--and that is a major flaw, because with all its ensembles this is very much a conductor's opera. The orchestra was simply out of synch too much, and given the quality of the ensemble singing that was a little puzzling.

The production sometimes was a little too broad in its humor. On the whole, horse-play doesn't really belong in Mozart, though Despina is an exception. Tonight's cast will be a different one.

What makes this production work as well as it does, above all, is its respect for Mozart's intimate sensibilities. Great voices are wonderful, but they aren't everything.