A Romeo and Juliet story not based on Shakespeare? That may be unthinkable in 21st-century America, but the Bard was virtually unknown in Italy when Bellini composed "The Capulets & the Montagues" in 1831. Felice Romani based his libretto instead on the Renaissance sources that inspired Shakespeare, and there are significant differences.

In the Pittsburgh Opera production that opened at the Benedum Saturday, be prepared to see Tybalt as Juliet's fiance, ready to avenge the Capulets after Romeo has killed their son in battle; and Romeo as a "trouser role" for a mezzo-soprano. Curiously, the practice of composing teenage boy parts for a female voice, not uncommon in the 19th century, is the reverse of Shakespearean custom, where women were not allowed on stage and men took the female roles. Bellini later adapted his Romeo for a tenor, but that's a moot point when the hero is portrayed with the consummate artistry of Vivica Genaux, and Giulietta (Juliet) by the supple-voiced, willowy Laura Claycomb.

With Thor Steingraber's cleverly updated staging, the show takes on a "Masterpiece Theater" aura that works to advantage throughout. No less critical are Robert Israel's minimalist set (using cartoon-like backdrops and a few strategic chairs), his elegant costumes and the effective lighting designed by Christine Solger-Binder. Conductor Antony Walker maintains the simplicity of Bellini's writing with tidy orchestral work that allows the singers leeway to convey their requisite sentiments. Together, they turn what might have been a mere tour de force for two superb singers into a totally magical evening.

Bellini was a composer of the so-called "bel canto" school, and the singing of the two protagonists was just that. The term, which literally means beautiful singing, has come to denote a style of opera that emphasizes voice above all other elements, as well as the technique of singing developed in Italy in the 18th and early 19th century. The plot moves forward in declamation (recitative), but attention is focused on elaborate arias featuring a slow cavatina that shows off the singer's ability to spin long-drawn melodies over a simple accompaniment. A fast cabaletta with elaborate vocal fireworks usually follows.

When Genaux and Claycomb sing their big duet -- Romeo unsuccessfully urging the timid Giulietta to run away with him -- their combined sounds are vocal heaven. The melodies are so beautiful in themselves, that Bellini was able to recycle some of them from his now-obscure earlier operas. It's up to the singers to infuse them with new meaning and emotion, and the present duo is expert at that. They provide an evening of gorgeous singing, appealing as pure sound but pertinent to character and drama.

Genaux gets the first chance to shine, entering Capulet territory disguised to plead forgiveness on Romeo's behalf. Genaux has a unique sound, quite unlike any other singer in memory, and her delivery of Romeo's opening recitative contains authority and a range of color from vibrant metallic low notes to thrilling, strong highs. The pleading lines of "Se Romeo t'uccise un figlio" (If Romeo killed your son) give way to a thrilling delivery of the cabaletta avowing a fight to the death for his love. In the final scene at the tomb, Genaux makes Romeo's grief all but unbearable.

Claycomb, at her entrance, has the opera's most familiar extract, the exquisite cavatina, "O quante volte," musing on how many times she has asked heaven for her Romeo. It's a haunting melody, and this remarkable soprano shaped the phrases with infinite nuances of meaning. Claycomb's light soprano is flexible yet penetrating, her technique unfailing in accomplishing her dramatic and musical ends. She was even more moving in the later, less-known aria where she begs her unrelenting father for a final embrace.

Among the three supporting characters, only Tebaldo (Tybalt) has an aria. Opera Center alumnus Arthur Espiritu delivered it enthusiastically, his lightweight tenor having an impressive high extension. David Pittsinger's booming bass voice was an asset for Capulet, as was Jonathan Beyer's sweet-toned baritone as Lorenzo (a counterpart of Shakespeare's Friar Lawrence). The chorus prepared by Mark Trawka was an integral part of the drama, singing with sharp precision and resonant tone. It's all male in the score, but Steingraber has added women as extras to clarify some of the action.