Two complete casts of singers were lined up for the Washington Opera's production of Vincenzo Bellini's "I Puritani," which opened Dec. 29, and the casts were supposed to alternate nights throughout the run. But a flu bug ravaged the company, and not until Monday did the entire "B" cast sing together.

But even then things didn't unfold as planned; most of the singers sounded congested or recovering. The romantic leads who started the evening, soprano Eteri Lamoris as Elvira and tenor Alexandru Badea as Arturo, seemed out of sorts during Act 1. Arturo, a Royalist who abandons his love for a political cause, is on the lam from Cromwell's Puritan authorities for all of Act 2 and has nothing to sing. This gave Badea a chance to rest, and he stayed on until the opera's conclusion. He managed soft, steady lines without apparent strain, but all his high notes sounded effortful at best, uncomfortably bitter at worst.

Lamoris called it a night after the first act, and following a slightly lengthened intermission, she was replaced by the "A" cast Elvira, Lynette Tapia, an agile, small-voiced soprano with a pinched, nasal tone and girlish inflections. Elvira suffers multiple mad scenes, but Tapia didn't distinguish them from her character's non-crazy behavior.

Bass Rosendo Flores as Elvira's uncle Giorgio and baritone Guido LeBron as her jilted suitor Riccardo were perhaps the heathiest of the bunch, and their voices blended nicely. Their duet to close Act 2, "Suoni la tromba," was hearty, graceful and the best thing heard all evening. Jason Stearns, as Elvira's father, and Stephanie Novacek, as the dethroned queen, held their parts firmly, but, again, it was difficult to know how illness compromised the singers' voices. Christopher Larkin conducted the Kennedy Center Opera House Orchestra with clarity but little intensity.

It's anyone's guess which singers you'll hear if you catch one of the four remaining performances, all at the Kennedy Center's Eisenhower Theater.