They'll come by bus and truck - the 18 singers of the Western Opera Theater, prepared to mix and match in four complete casts of Verdi's "La Traviata."

Western Opera Theater is the touring arm of the San Francisco Opera Center and the only touring opera company in America today. Its performances in five Utah communities, ranging from Logan to Cedar City, are scheduled as follows:Thursday, Jan. 9: Kingsbury Hall, University of Utah campus, 8 p.m., $16-$18 general public, $11-$13 students.

Friday, Jan. 10: Kent Concert Hall, Utah State University in Logan, 7:30 p.m., sponsored by Cache Performances, $12, $7 students.

Saturday, Jan. 11: de Jong Concert Hall, Harris Fine Arts Center, Brigham Young University, Provo, 7:30 p.m., $10, $8 students.

Sunday, Jan. 12: Egyptian Theatre, 7 p.m., sponsored by Park City Performances, $25, $35.

Wednesday, Jan. 15: Cedar City High School auditorium, 7:30 p.m., sponsored by Cedar City Music Arts, $10.

The tour is jointly sponsored by the Utah Arts Council, as part of its 1991-92 Performing Arts Tour.

Singers are not regularly announced in advance for specific performances, but the Salt Lake cast has been selected - Diane Alexander as Violetta, Stephen Guggenheim as Alfred and Zheng Zhou as the elder Germont. Elsewhere, roles will alternate among young professionals who represent the cream of the crop at the Opera Center. All performances will be sung in English.

WOT performances are fully staged and costumed, with skilled technicians behind the scenes. The company also includes a music director/conductor, in this case William Vendice, of the Metropolitan Opera staff; coach/accompanists, full wardrobe, production and makeup assistants. The group tours with more than 15,000 pounds of scenery, lights and costumes. Performances are given with two-piano accompaniment.

The company is presenting "La Traviata" for the first time in its 25-year history. In some places they will also give "Operatunities," special educational workshops tailored to the needs of the community.

Since Kurt Herbert Adler founded the company in 1967, Western Opera Theater has given more than 1,500 performances of 37 operas before more than 2 million people across the United States.

During its first season, the company sang "Cosi fan tutte" in Arizona, Nevada, Oregon and California - 22 performances in gymnasiums, high school lunchrooms/cafeterias and community theater facilities. Each year the company became stronger and more confident, and by the 1970s it was widely featured in the 12-state western region. With the support of ICM Artists, Ltd., the company now tours the east coast as well. In 1987, WOT became the first professional opera company to perform in the People's Republic of China.

WOT utilizes the SFO Center's national auditions and training programs (Merola Opera Program, Brown Bag Opera, Adler Fellowships and Showcase) to identify and prepare singers, music staff and technicians for its national tours. The company now averages four to six months annually on the road, presenting no less than 40, and as many as 65 performances.

Runs of seven nights in a row in diverse locations are not uncommon, said Diane Alexander, lyric coloratura from Rhode Island.

"We all make the whole tour together, and we never get a night off, we either sing a principal role or in the chorus," she said. "There are six women in all, and everyone is responsible for knowing three or four parts. The heavy schedule has taught me a lot about pacing myself, maintaining good health and a happy attitude."

The company's fall tour was difficult, with stops in 19 widely-scattered states ranging from Florida to Washington, New York to California, and long hops between performances. The winter tour will be easier, with performances closer spaced in California, Arizona and Nevada as well as Utah.

"Fortunately there are only 22 of us on our bus, so we each have our own seat to lie down on," she said.

Almost as important as musical ability among artists is social compatability, with the pressures of touring, and "this group is compatible, we have a good time together," she said. "But we are conscious of what we have to do, very careful how we spend our free time, and the person doing the lead that night is usually pretty quiet. The speaking voice is different than the singing voice, and I can sing longer than I can talk. But you save yourself, especially for Violetta. She is on for all but about 15 minutes of the two and one-half hour show."

Alexander comes from a musical family in Warwick, R.I. Her mother teaches in a private music school and is organist at the Calvary Baptist Church, where Diane began singing in thechoir. Her father also sings, but not pro-fessionally.

She began lessons when she was 15, and had her best training at Rhode Island College under Mary Beck, with whom she still works out.

Alexander is popular in Rhode Island and the Boston area, where she has "lots of supporters." In that small state she's worked with many groups, including the Providence Opera and Rhode Island Philharmonic and a community Gilbert & Sullivan company. She's sung in Sarah Caldwell's Boston Opera chorus and attended Tanglewood Music Center for a workshop on song literature with Phyllis Curtin.

After the WOT tour, Alexander looks forward to singing a pops concert with the Rhode Island Philharmonic and premiering "Vocalisms," a song cycle composed for her by Sebastian Currier, with the New Music Ensemble of Rhode Island.

As a New England winner of the Metropolitan Opera auditions, Alexander sang on the winners' broadcast from the Metropolitan stage, but finds singing in San Francisco "more comforting."

She recently studied in Siena, Italy, for a summer and "I can't tell you how it changed me as an artist, walking the streets of Florence, and knowing that's where opera originated. Such a feeling! I began to prepare my Violetta there."

Jill Blalock of Lubbock, Texas, toured last year in WOT's "Lucia," but finds this tour easier because the stops are closer together. "La Traviata' is more popular," she observed.

"It's a grind, driving for six hours, then taking a two-hour break and getting up to do a show. But the tour satisfies my sense of adventure; I see places I'd never see otherwise and meet a lot of nice people.

"And singing Violetta is such a wonderful opportunity, it's every lyric-coloratura's dream to perform this role, but you seldom have a chance to do it many times over like this. I never get tired of singing Violetta, she has so many exciting facets, such a complex character."

Blalock's parents play piano and sing, but they didn't like opera until Jill began to sing it. Like Alexander, she began singing in her own Baptist Church choir, studied with Mary Gillis in Lubbock and attended Texas Tech there. She still works out with Gillis when she's home.

She spent two years studying in Houston, then did the SFO Merola program two years ago and has sung in regional opera companies. "I've also sung with the symphony in my home town and done some things in Dallas, where people know who I am," she said.

Blalock looks forward to singing a Lucia in Fort Worth, a summer concert in Ohio, and a production of "The Merry Widow" at the June Opera Festival in Princeton, N.J. She's also sung twice with Eugene (Ore.) Opera.

She was recently married to a singer she met on tour last year. They live in New York, near their agents and audition opportunities. "Almost everyone comes to New York to audition, I have an audition lined up with the Mannheim (Germany) Opera," she said. Quite candidly, Blalock finds the present outlook for singers "not a great one. Work is hard to come by, with budget cutting everywhere."

Among the many singers who have progressed from WOT to prominent careers are Allan Monk, Pamela South, Kevin Langan, Cheryl Parrish, Ruth Ann Swenson, Nancy Gustafson, David Malis, Dolora Zajick, Barbara Kilduff, Deborah Voigt and Ann Panagulias.