The rare 18th Century violin of the New York City Opera's concert master, which had been missing for six days, turned up yesterday in a remote storage area of the Kennedy Center Opera House.

It was discovered a few minutes before yesterday's matinee performance of Puccini's "La Boheme" in a dark, out-of-the-way place under the stage. The instrument was behind some rows of seats that had been removed from the auditorium to make space for a larger orchestra pit.

The 1757 Venetian instrument had been missing since last Sunday when John Pintavalle had discovered the instrument was gone from his open locker in the musician's lounge. It had disappeared during the two-hour interval between the matinee and evening performances. Such an event had never occurred before at the Kennedy Center and such events are rare anywhere.

Park Police are continuing to operate on the assumption that the loss was a theft and will pursue the investigation, a spokesman said yesterday.

The euphoric Pintavalle described yesterday how the violin was found: "Lots of us had been combing those recesses of the center for days, with no luck. But yesterday, just as we were all arriving, and I was sitting in the lounge, Shirley Slegelman, one of the other violinists, came running in and said she had found a case in the storage area.

"Our first instinct was that somebody had just accidentally left an instrument there, and then it occurred to both of us that the case might be mine. But I had looked in exactly that spot two days ago and nothing was there. A number of others had searched the same area.

"I walked in and there it was - it was obviously my case, with its canvas and heavy leather underneath. So I held my breath and opened it.

"Everything was there and everything was in place. My guess is that the case had not even been opened in the days it was gone.

"Shirley broke into tears and you can imagine how I felt."

It was discovered in time so that those who heard yesterday's "La Boheme" were hearing the opera's frequent violin passages played on Pintavalle's recovered violin, which he values at about $50,000. It was the instrument on which he has performed almost daily since 1964, and it was also the final day of a most anxious week.