The great tenor Enrico Caruso sang about 60 roles; the storied diva Maria Callas, roughly 50. Renée Fleming, the most famous soprano today, says she has sung about 55.

But Plácido Domingo has blazed past them all. When he recently took the stage for a concert performance of Bizet's "The Pearl Fishers" at the Salzburg Festival in Austria, he reached a virtually unheard-of milestone in opera history: He sang his 150th role.

"If you look at the history of singers in opera, he stands by himself," said Joseph Volpe, former general manager of the Metropolitan Opera. "If there was ever a giant in any industry, it's Plácido Domingo. He's unmatched."

At 77, well past the age at which most star singers retire, Domingo has performed nearly 4,000 times in a six-decade career, recorded more than 100 albums, and become a household name as one of the Three Tenors, not to mention appearances on everything from "Sesame Street" to "The Simpsons." And he has continued to add voraciously to his repertoire, choosing roles to match his changing voice, while also becoming a prominent conductor and arts administrator.

As early as the 1970s, he was told to slow down or risk burnout. (In a 1972 New York Times Magazine profile, Callas told him, "You're singing too much.") As he entered his 60s, then his 70s, critics and peers repeatedly suggested he should retire.

Nothing has stopped him. "When I rest, I rust," he said in an e-mail.

When his tenor high notes began to give out, he moved down to baritone roles. Domingo's debut in his 149th role, as the baritone Miller in Verdi's "Luisa Miller" at the Met last spring, was sold out — as are most of his performances around the world — and widely praised.

"He's a legend," said Peter Gelb, the Met's general manager. "If you're a baseball fan, who wouldn't want to see Babe Ruth in the final years of his career? And, like Babe Ruth, Domingo has delivered."

Domingo, who was born in Spain but moved to Mexico with his zarzuela-singing parents as a young boy, swiftly rose to fame after what he considers his debut, in Verdi's "Rigoletto," in Mexico City in 1959. Within a decade, he arrived at the Met unexpectedly, filling in for an ailing Franco Corelli in Cilea's "Adriana Lecouvreur" alongside Renata Tebaldi.

From there, he deftly navigated new roles across diverse operatic styles, from Wagnerian weight to bel canto lightness. Among his signatures was Verdi's "Otello"; Fleming, who sang Desdemona with him in the 1990s, said that even when she was supposed to be playing dead onstage, her "tears were running" at the beauty of his voice.

His only allowance for aging is working harder.

"I can tell you that it's much harder to memorize text and music now than it was 30 or 40 or 50 years ago," he said. "But I won't let that stop me."

Only after internalizing the score does he begin to sing in earnest. This, he said, has helped throughout his career to conserve his voice and energy. (Other habits, he added, include nothing out of the ordinary: sleep, only occasional drink and a balanced diet without too many sweets, which he loves.)

He also doesn't talk much the day before a performance. Gelb recalled Domingo sitting in his dressing room, somber and more nervous than usual, before the Met's live broadcast of "Luisa Miller" this spring. Domingo said he'd been up since 4 a.m., praying the performance would be a success.

Don't even talk to him about slowing down. In the coming season, he plans to unveil his 151st role, in Manuel Moreno Penella's "El Gato Montés" at Los Angeles Opera, where he is general director. He has a new album with Spanish guitarist Pablo Sáinz Villegas and is scheduled to return to the Met in November as Puccini's Gianni Schicchi, to celebrate the 50th anniversary of his debut there.

Domingo still has singing at the center of his life. "I want to do it," he said, "as long as I can do it well."