Mikhail Baryshnikov, the expatriate Russian ballet star, is leaving the American Ballet Theater to join George Balanchine's New York City Ballet, it was learned yesterday.

Baryshnikov, 30, is the current matinee idol of ballet, and in fact was nominated for an Academy Award upon his screen debut last year in "The Turning Point."

He is leaving a company that emphasizes individual stardom among its principals, however, for a troupe which not only soft-pedals the personal reputations of its dancers but makes a point of listing its company roster in strict alphabetical order.

There is no word yet as to whether Baryshnikov, whose own productions for ABT of "The Nutcracker" and the recently premiered "Don Quixote" made their bow at the Kennedy Center, will continue his choreographic activity at the New York City Ballet.

His move, however, comes at a time when Balanchine, still curtailing his exertions after a recent hospitalization for heart trouble, is cutting back on his creative work. Balanchine, who is 74, was to have completed his long-promised trilogy of "national" ballets by adding a new "Tricolor" to his earlier "Stars and Stripes" and "Union Jack."

Earlier this week, however, the company announced that Balanchine would only supervise Tricolore," and that the actual choreography, to be unveiled at a gala May 17, was to be assigned to company principal's Peter Martins and Jean-Pierre Bonnefous, [WORD ILLEGIBLE] with City Ballet's co-ballet master, Jerome [WORD ILLEGIBLE]

For ABT, Baryshnikov's move will mean the loss of one of its supreme attractions for a company which afforded him his first haven in this country. ABT co-director Lucia Chase said yesterday.

"When Mischa Baryshnikov joined American Ballet Theater about 4 years ago, it was a time of enormous challenge. In the years that Mischa has been with us, we have experienced some of the greatest moments in ballet history. As a dancer he has brought his enormous artistry, his sensitivity, his power to the stage, and he has been an example to everyone in the company. He has also given us two magnificent productions of his own which he and the public will enjoy for many years to come.

"Development is a key word in Mischa's life . . . To have the opportunity to dance the works of George Balanchine and Jerome Robbins, with their direct supervision, coaching and inspiration, is a creative experience of the highest order. We wish Mischa the best, we wish him our love and support in this decision, and we hope we will all work together again."

Balanchine was unavailable for comment yesterday but a New York City Ballet source said that Balanchine had accepted Baryshnikov's recent overtures to join the company. The actual move will occur when Baryshnikov is done with his present commitments, probably toward the end of June. He will be with the company for its annual summer residence at Saratoga Springs, July 5 to 22, though no specific performances of his have yet been scheduled.

His name will be listed, the source said, in alphabetical order, just like any other company meber, and his status will not be that of a "guest artist" but of a "permament" company member.

The switch of companies for Baryshnikov may give the New York City Ballet, which visits the Kennedy Center now annually for two or three weeks, even greater visibility than before, but may also mean less Washington exposure for Baryshnikov, who has been appearing here regularly with ABT during its seven weeks of performances at the Kennedy Center Opera House.

Commenting on the development, Martin Feinstein, Kennedy Center executive director, said yesterday: "On behalf of ABT, we are very sorry to lose him, but on the other hand Kennedy Center ballet subscribers are very fortunate to get both the New York City Ballet and ABT, so our subscribers won't lose him. I'm sure the ABT management realizes what the loss means and I'm also sure they'll come up with many interesting solutions to make up for it."

Since Baryshnikov's defection four years ago, there's been speculation in ballet circles about the potential fusing of the talents of Baryshnikov, the outstanding classical virtuoso of the day, and Balanchine, the acknowledged choreographic genius of out time.

Both men have publicly expressed interest in the possibility but downplayed the likelihood of its coming to pass. Baryshnikov, however, fueled the specualtion his book "Baryshnikov at Work," published in 1976, by stating that "dancing (Balanchine's) Theme and Variations' was the realization for me of a dream . . . I had seen several Balanchine works, and I knew an know what I wanted to dance as many of them as possible . . . I have much more Balanchine to dance before it is all over."