Wagner onced flipped through the pages of Bellini's "I Capuleti e i Montecchi" and pronounced it: Not bad, with the right soprano. Typical Wagnerian condescension. But he was right about the necessity of a top-drawer singer to sing Giulietta, a role that demands the full resources of a substantial coloratura voice, and the ability to project into the music contradictory qualities of vulnerability, youth and moral vigor.

The Washington Concert Opera, which performed the work Sunday at the Kennedy Center Concert Hall, found just the singer. Sumi Jo, a Korean soprano at the forefront of the opera world for more than a decade, triumphed in the role. Although she has recorded parts of the work, this was, according to the WCO, her first performance of the complete opera.

The astonishing results bode well for the soprano. Jo has been prey for years to the usual critical canards that dog coloraturas--that the voice is pretty but colorless, the interpretation usually shallow. Perhaps she has lacked some of the necessary weight and emotional depth to sing Donizetti's Lucia and Bellini's Elvira, but Giulietta is now a role she can own, with few, if any, rivals today.

The role works for her because it is inherently tentative, with flashes of fire emerging like brief sunbursts. In terms of the drama, this retelling of the Romeo and Juliet story has nothing to do with Shakespeare, except that Bellini's librettist went back to some of Shakespeare's original sources. In re-creating the story, he made the central characters much more involved in their own destiny, turning the tale into a typical bel canto meditation on love and duty. Bellini's Giulietta isn't just young and in love, she's torn by, and engaged in, the conflicts of the two warring gangs of thugs.

Unlike many other bel canto soprano roles, at no point in the opera does Giulietta come entirely unglued, which may make it more temperamentally suited for Jo. Each scene is a succession of highs and lows, hopes and fears. Sustained emotional dissolution, difficult for any actress, is never called for.

On the other hand, sustained flights of the most delicate and sensitive lyrical singing definitely are, and Jo's control over this aspect of the music was impeccable. In her first-act romanza she compares Romeo's sighing to the wind coming through her window; the music, her feather-light and nuanced line, made the comparison musically explicit.

Jo was fortunate to be partnered by mezzo-soprano Theodora Hanslowe (Bellini wrote Romeo to be sung by a mezzo-soprano). Hanslowe, too, made Romeo a bit hesitant, conflicted between his martial self-confidence and his craving for peace and eros. Hanslowe never attacked her lines in a futile effort to make them sound more masculine; but by carefully marshaling her vocal resources (which are manifold) she created a long, evening-length dramatic arch that established a range of emotional growth that made Romeo believable.

Hanslowe and Jo have well-matched voices, especially for an opera that requires sustained close singing in its duets. Together they mastered the spine-tingling illusion of two individual voices, each with a separate character, yielding magically to the perfection of two identical instruments playing a parallel line. There were moments in both acts of the opera when everything inessential--plot and character and time itself--disappeared, replaced by pure expressivity, hanging in the air for a few sustained and blissfully unreal moments.

Jo and Hanslowe stole the show, but three male singers supported them with professional contributions. Tenor Jorge Lopez-Yanez sang the obligatory tenor role (Tebaldo) with enough force to cut through the massed chorus and orchestra; it was a neutral presence and one could hear Bellini's disinterest in the role. Stephen Morscheck was a rigorous and appropriately stern Capellio, with snappy diction and precise, clipped musical phrases. Eric Owens, as Lorenzo, had little to do. Saturday's performance left off the interpolated final scene (written by Vaccai), which makes Lorenzo a more substantial presence. Still, Owens helped ground ensemble passages with a pleasant and plangent bass-baritone.

"I Capuleti e i Montecchi" has never taken its place beside the composer's "Norma," "La Sonnambula" and "I Puritani." Historians frequently echo Wagner, finding much to admire but little to love. Perhaps it gets a bad reputation because the composer cannibalized early works for much of the hastily completed score, or because no single moment stands out with shattering impact. But it is one of Bellini's most concentrated and deeply felt music dramas, which the WCO, under the able direction of Stephen Crout, proved again over the weekend.