Marta Istomin, a musician and the wife of Eugene Istomin, one of the world's most celerated concert pianists, has been appointed artistic director of the Kennedy Center, officials there announced yesterday.

Istomin, 43, a native of Puerto Rico, will begin immediately in the new position.

"She's not going to be a replacement for Martin Feinstein," said one Kennedy Center spokesman. Feinstein has served as executive director of performing arts at the Center until last September, when his position was abolished. Since then, the Washington arts community has been waiting for word of a replacement for the controversial figure. But Istomin will assume only a part of Feintein's former responsibilities.

Istomin will work with other members of the Kennedy Center staff to develop programs in the performing arts -- excluding theater, which is the domain of Center chairman Roger L. Stevens. Many people and organizations already plan events at the Kennedy Center. Currently, the National Symphony Orchestra and the Washington Opera Society, both directed by Martin Feinstein, perform there. And the Washington Performing Arts Society, another separate entity, headed by Patrick Hayes books many performers into the Center.

"The Kennedy Center has a major coordinating role however," Center director of operations Thomas Kendrick said yesterday. "She would obviously coordinate with WPAS and other organizations about booking, and about sharing of financial responsibilities for some events. In addition, she would be involved in planning events ranging from music festivals in the concert hall to chamber music and dance in the Terrace Theater."

"I have to get acquainted with the environment and the institution," Istomin said yesterday. "The Center is a cooperative venture."

Asked what type of activities she would like to plan, she said, "My area is in the musical arts. There will be a proper balance [of events]. I have to study the whole workings of the Center."

Istomin said she was asked to take the job three months ago: "It's a high honor to be asked to serve the arts in the capital of the U.S. In all candor, I had not thought of aspiring to this position. The position sought me out."

She said she consulted with Feinstein, "a friend for many years," before taking the job. "We talked about it a lot." She accepted because of "the pride and the thought of holding the office as a Latin American, a woman and a Puerto Rican, and also because of the opportunity to serve the arts."

Istomin will find an apartment in Washington but will also keep the New York apartment she shares with her husband, who is almost constantly on tour.

"That's one of the sacrifices I make for this job -- not touring with him," said Istomin. "I usually go with him on these tours. But you can be assured we will make every effort to be together. He's very happy that I will have this opportunity."

Marta Istomin is the former wife of the late cellist Pablo Casals, whom she married in 1956 when she was 20 and he was 80. She was extremely active in the development of the arts in her native Puerto Rico -- where she and Casals brought the Casals Festival, and together established the Puerto Rico Symphony Orchestra and the Conservatory of Music.

Istomin is a cellist, but has not performed since 1961. She first met Casals in 1954 when whe won a $1,000 prize to study with him in France.

Currently, she sits on the board of Harcourt Brace Jovanovich, Inc., and consults for the publishing house on educational projects in Latin America. She is a director of the World University of Puerto Rico and a trustee of both the Marlboro School of Music in Vermont and the Marymount School in New York. She is also co-founder and vice president of the Pablo Casals Foundation in Barcelona, Spain.

"He background is a consideration -- obviously," said Kenderick. "It's a national cultural center and you want someone whose credentials are established."

The search for personnel in artistic direction of the Center started long before Feinstein left, Kendrick said.

"There has been a need for additional artistic personnel," said Kendrick. "There may be other people in theater or dance brought in to work with Marta or Roger. But Marta is a major appointment."

Center chairman Stevens could not be reached for comment late yesterday afternoon.