WHEN Chrissellene Petropoulos was 9 years old, her fourth-grade teacher gave the class an assignment to write the stories of their lives. She wrote, in a thin paper book, which her mother still has, "When I was 5, I loved to sing opera! I lived opera! I love opera!"

In two weeks Petropoulos will be 27. Four weeks after that she will fly to Vienna to take up a new life with the Vienna State Opera. The little girl who grew up in Potomac, Md., went to Winston Churchill High School and the University of Maryland -- and who became a singer by chance -- has landed a contract with one of the most prestigious opera companies in the world.

Moreover, in addition to studying in the State Opera's Studio, she also will sing in 30 performances during the coming season, with the title of "Solo Singer," and with the accompanying higher pay.

All summer Petropoulos has been taking daily German lessons, getting ready for Vienna and the big house where German is the bread-and-butter language. "Going to live in Vienna and to work in that opera company is the biggest honor of my life," she says. "I don't know just where I am going to live but it will be within walking distance of the Staatsoper so that I can go to the opera every night."

Asked how she got such a contract, she says, "When I was little, I played basketball, football, soccer," as if those were the paths to operatic stardom. But, of course, there was music: "I began to study piano when I was 4. With Miksa Merson.When my mother took me to him, he said, 'I don't teach children!' But while they were in the next room, I sat down at the piano and began playing 'Chopsticks.' He asked who was playing like that, and then he said he would teach me."

As if the piano were not enough, Petropoulos says that one day, "when I was 5, I saw an orchestra with all those violin bows going up and down, and I said, 'I want to do that.' So when I was 5 to 6, I started on the violin too," with Neva Greenwood.

All this music study did not persuade Petropoulos that she should be a professional musician. Her father is a physician, and although she says, "I could never be a surgeon," she was "interested in people and in psychology. I should have been a psychiatrist with a couch."

By the time she entered the University of Maryland, she had decided to major in some field of music and psychology. One day her father suggested that music therapy was the ideal combination of the two. In the process of meeting the requirements for the music side of that degree, Petropoulos found out she had a voice.

As a junior in 1975, she had decided that she did not want a performing major (that is, either as a violinist or a pianist). So the committee that was holding auditions for admissions told her to "sing something." When she said she had no music with her but could sing, "My Country 'Tis of Thee," they gave her a score of Handel's "Messiah" and said, more or less, "Go upstairs and work on that for a while and come back and sing it." Protesting that she did not know beans about singing, the future member of the Vienna State Opera -- who to this day has sung exactly one leading role in opera -- and who calls herself a "Soprano Brand X" -- did exactly as she was told and was admitted.

Four months after she became a voice student of Leon Fleming, Petropoulos came up to her first faculty audition as a singer. "That's when the surprises began," she says today. "I had thought, 'Okay, I can carry a tune; I'll get a degree in voice.' But after four months with Mr. Fleming, people began to talk about my becoming a singer."

Small wonder that she gives Fleming great thanks and credit for giving her a solid foundation and for pointing her in the direction she has followed ever since. She says, "He made me learn songs, repertoire. He got me ready for a senior recital, said it would be good for me to do it even though it was not a requirement for my degree. Before long he began talking about where I should go to graduate school. He thought the Curtis Institute in Philadelphia would be the best place, and so a year before I graduated from Maryland, I went up to Curtis to audition.

"The whole voice faculty was there to hear each of us. When I was through, Margaret Harshaw spoke up and said, 'I want you.' When I told them that I still had another year to go to finish my degree, they said that was okay, to go ahead and get it and then, when I was ready for Curtis, to come back -- I would not have to do any more auditioning."

That same summer of 1975, Petropoulos created something of a sensation with her singing of "Piangero" at the convention of the National Association of Teachers of Singing of the Maryland campus. The following year, with her B.A. safety tucked away, she moved up to Philadelphia and began her work at Curtis.

In her first year there, she was told to prepare the title role in Massenet's "Manon." "But I don't want to sing 'Manon!'" she told a startled Sol Schoenbach who had proposed it to her. "I can't sing a major scale and you want me to sing a major role!" Schoenbach said it was the first time he could remember any singer who was offered a major role turning it down in order to sing a minor part. But he had not met a Chrissellene Petropoulos before. She was following the advice of some trusted friends who had urged her to take things slowly. At Curtis, she sang minor roles -- a nun in Puccini's "Suor Angelica" and the child Yniold in Debussy's "Pelleas et Melisande" -- before taking on her only major role thus far, Mozart's Despina in "Cosi fan tutte."

In the summers Petropoulos followed Harshaw out to the University of Indiana at Bloomington -- where Harshaw also teaches -- to spend long, concentrated hours. Harshaw told her, "Instinct is not enough. We have to build a solid technique. How do you breathe?" When Petropoulos admitted that she "just breathed," Harshaw explained the inadequacies of so simple a system and intensified their work on technique. One day Harshaw asked her pupil, "Can you trill?" Petropoulos answered, "Yes, if it's like it is on the piano, I can do it," and did. She had a natural trill -- some do, some don't.

Back at Curtis for her second and final year, she sang her first full recital with Sokoloff as her pianist, sang for the first time with orchestra, and finally sang the Despina. When she was assigned that role, she says "They told me 'You are a Despina.' So I walked around for days and weeks asking myself, 'How would Despina walk? What would Despina do? How would Despina go into a store and up to the candy counter?' I thought Despina until I made myself become Despina."

After two years, Harshaw told Petropoulos she thought she should go and work with someone else. Coming back to Washington, she was urged to study with Todd Duncan. Petropoulos today speaks of him in language usually reserved for descriptions of the ancient deities.

"I think Todd Duncan got me into the Vienna State Opera," she says, with love in her face."He became a god for me. He talked about deep breathing, about how it was never deep enough. And his loving handling of the voice. He made me work twice as hard. And he has such a desire to give his students the very best he has. His knowledge of technique is incredible. He was everything I needed." And so he seemed to be, because it was while she was working with Duncan that Petropoulos sang in some major auditions.

One day last February Luciano Pavarotti was hearing a dozen young singers in Carnegie Hall. One of them was Petropoulos. After all 12 had sung, the great Italian tenor said, "I want to hear Petropoulos again," and began some brief, intensive coaching. "Sing me 'Caro nome.'" After she finished the aria he asked, "Can you sing this phrase this way?" -- to which she gave her accustomed reply, "Well, I can always try." And out it came the way Pavarotti had suggested. "Now can you do it this way?" he wanted to know, demonstrating a different kind of phrasing. And again she sang what he asked for.

Another time Petropoulos was singing in a master class given by the late Walter Legge and his wife, soprano Elizabeth Schwarzkopf. When she sang a Greek folk song, Legge -- the mentor of Callas, Schwarzkopf and others of the world's great singers -- told her, "Young lady, do you know that you can have a career or just that song!" That was not the last time that "To Layoarni" was to make its mark on Petropoulos' hearers.

A few weeks ago the great Brazilian star, Bidu Sayao, was conducting a master class at the Peabody Conservatory in Baltimore. Petropoulos was the last to sing for her, held back by those in charge of the class. For her final song, she told Sayao she wanted to dedicate a song to her, and sang "To Layoarni." When she had finished, Sayao, with tears in her eyes, told Petropoulos, "That is singing. When you have your German, your French and your Italian where your Greek is, you are in."

Sayao then asked Petropoulos to sing "Ah non credea mirarti," from Bellini's "La Sonnambula." Saying that she had never sung it before, Petropoulos sang it for the exquisite star of more than 200 Metropolitan performances. When it was over, Sayao said, "You say you have never sung it before? But there is nothing for me to tell you!"

One day last October, when the Vienna State Opera was in is famous visit at the Kennedy Center, there was an audition. Conductor Karl Boehm was there and so was the noted soprano Leonie Rysanek and her husband, Elu Gausmann, a musicologist and opera authority. They were listening to the promising young soprano from Potomac.

When the audition was over, Petropoulos' mother began to tell Boehm, "Now, if you say she is not good enough, that's all right. She can teach school." Boehm simply said, "If I say she is good, she is good. And I say she is good. She has a lovely voice -- and everything to learn."

It was settled. Petropoulos was to report to the Vienna State Opera Studio for coaching in all phases of opera. In March she flew to Vienna and went to work. In June she and the other students were required to sing for the full committee of the State Opera on the stage of the famous house on the Opera Ring. As she had always done, Chrissellene wore a long dress. When she finished singing, someone on the committee -- which again included Rysanek as well as Hilde Gueden, one of the unforgettable greats of Vienna, Emmy Loose, and others -- said. "Let us see your legs."

"My legs?" asked Petropoulos.

"Yes, your legs. We have to see how they look on the stage."

Well, there is nothing wrong with the Petropoulos legs, and it is now likely that they will be seen with increasing frequency on some of the world's major opera stages before long. Because that contract arrived a couple of weeks ago with its unexpected clause about "30 performances during the season," in addition to the studies, and that title "Solo Singer."

Loose, one of the most polished and admired artists of the post-war era in Vienna, said after the audition, "This is a voice I can do something with."

And Petropoulos said, after one lesson, "I knew she was for me."

Somewhere in the back of Petropoulos' head there must be echoing the comment made to her by one of the Viennese singers after her final audition: "What you have cannot be learned or taught." What can be taught, Petropoulos is setting out to learn. It seems likely that the public will be hearing more of her before long.