Saturday afternoon Sarah Beth Goncarovs, who is 9 years old, will take her place behind an immense Yamaha concert grand to perform a Mozart concerto with the Alexandria Symphony.

Although she has played with youth and chamber orchestras on five previous occasions, this is the first time that Sarah Beth, who lives near Alexandria with her parents, Susan and Valdis, will appear with a full-fledged adult symphony orchestra. If the past is any indication, she will be quite comfortable in the role.

"She has kind of been an adopted child of the Alexandria Symphony for years and has played for our guild. I am delighted that we have an opportunity to collaborate, especially in a family concert," said conductor Kim Allen Kluge. The concerto will be featured in the symphony's first family concert, to be held at 5:30 p.m. at T.C. Williams High School.

"There will be a lot of {Sarah Beth's} peers in the audience," Kluge said. "She is a great role model -- very conscientious, and she really cares about her physical approach to the piano."

Kluge said he approached his young guest much the same as he would any guest artist. "My relationship with her was identical with any guest artist. We have a meeting, I hear her play. And then we have a discussion. She certainly was not intimidated by me in any way. She's very gifted, very musical and so endearing."

Shy, composed and undeniably endearing with her sad, dark eyes, Sarah Beth sat on her living room couch last weekend and talked quietly about herself and her music. While she talked, she worked methodically on a perfectly crafted green needlepoint tissue box decorated with black cats and pink bows. "I've been making boxes and purses," she said, and pointed to a small "bone box" that she created for the family dog, Broccoli.

She talked about her best friends, Joshua and Brenna, who live close by. She talked about sleep-overs, where "we get to bed around 11, unless it is one of my other friends like I'm making this box for, it's usually 1 or 2 -- she's pretty talkative." And about favorite toys. "I'm a Lego maniac," she said with a giggle.

Reading is also important. "My favorite book is 'Matilda' by Roald Dahl. 'Witches' is a very good book too. I'm trying to read everything he wrote." Other books? " 'Charlie and the Chocolate Factory,' and I like Sherlock Holmes. My favorite is 'The Speckled Band.' I read it every six months."

As for watching TV: "Not much. I'm usually doing something, working on needlepoint or watching a movie." There is a fine but critical distinction between TV and videotape. "The week I wanted to be an archaeologist, I watched Indiana Jones all the time."

And playing with her 3-year-old sister, Annie? "I don't like that."

"Ain't got a choice," piped in mom from the kitchen.

Susan Goncarovs said that her daughter's interest in the piano began when she herself started taking lessons as an adult. Four-year-old Sarah Beth asked her mom to write numbers on her fingers so she could find the notes. "Musicianship is in my family, but not the discipline. Sarah Beth has the discipline."

When asked, Sarah Beth played a favorite piece, "Birds," by Seymour Bernstein. "Each piece in the whole book is a different bird," she explained. She played from memory and with a confidence based on plenty of experience. Her physical gestures at the piano were fluid, graceful and totally in tune with varying moods of the eight short compositions.

Does she ever get nervous when she performs? "In concert I'd say it's kind of nervous," she admitted, and then explained what she does about it. "I think that there's only one person -- one person is the audience. I play better for one person." A sophisticated solution. Then she added, almost as an aside, "Sometimes vice versa, I don't know."

Sarah Beth, a fifth-grader in the gifted and talented program at Belvedere Elementary School in Falls Church, has been studying for five years with teacher Rosita Kerr Mang, and practices "one or two hours a day." It is a revealing number, not much more than the average child might practice and not nearly as much as a single-minded, single-talented Wunderkind would. And sometimes, just like most kids, mom has to remind her.

Mang described what makes this pupil special. "Whenever I think of a student, I look at their natural body. Sarah Beth was built to play the piano. She has wonderful tone-pads {on her fingertips}, a roundness and an elegance and a strong upper body. As a teacher, I look for natural dexterity -- it's either there or it isn't -- and she has it.

"And what I find fascinating at 9 is that she has this wonderful ear for color and dynamics. This is pretty special. She is elegant, aristocratic and so tasteful -- she is even tasteful in her clothes."

Mang encourages all her students to play in public and to enter competitions. Sarah Beth is a first-prize winner of the Washington Music Teachers Association Hartman Piano Performance Award and the Springfield Music Club Ramberg Competition, as well as a past winner of the club's Leetsma Competition. She has given a solo recital for the Lyceum Concert Series in Alexandria and was the guest artist of the 1989 Virginia Federation of Music Clubs annual convention in Norfolk. She will appear with the McLean Youth Orchestra next July.

Teacher Mang offered two words to sum up the musical personality of Sarah Beth Goncarovs: "Intimate warmth. But it doesn't come out because she's shy. She doesn't fill a room -- her music does."