Drummer Nick Menza, who played on many of Megadeth's most successful albums, died after he collapsed on stage during a show. He was 51.
Menza was into the third song with his new band Ohm when he collapsed late Saturday night, said Ohm's manager Steve Bauer. Paramedics couldn't revive him.
The band was playing at The Baked Potato, in Studio City, California.
"The group's bassist texted me last night after it happened," Bauer told CNN. "We are all still in shock."
Officials haven't released a cause of death.
Alongside Metallica, Slayer and Anthrax, Megadeth makes up metal's "Big Four." And for nine years in the early 1990s, Menza played on the band's biggest releases: "Rust in Peace," "Countdown to Extinction," and "Youthanasia."
He joined Ohm last year after Ohm lost its then-drummer David Eagle to heart complications. The band also includes former Megadeth guitarist Chris Poland.