Lesh's death was announced on his Instagram account. Lesh was the oldest and one of the longest surviving members of the band that came to define the acid rock sound emanating from San Francisco in the 1960s.
"Phil Lesh, bassist and founding member of The Grateful Dead, passed peacefully this morning. He was surrounded by his family and full of love. Phil brought immense joy to everyone around him and leaves behind a legacy of music and love," the Instagram statement reads in part.
The statement did not cite a specific cause of death and attempts to reach representatives for additional details were not immediately successful. Lesh had previously survived bouts of prostate cancer, bladder cancer and a 1998 liver transplant necessitated by the debilitating effects of a hepatitis C infection and years of heavy drinking.
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Although he kept a relatively low public profile, rarely granting interviews or speaking to the audience, fans and fellow band members recognized Lesh as a critical member of the Grateful Dead whose thundering lines on the six-string electric bass provided a brilliant counterpoint to lead guitarist Jerry Garcia's soaring solos and anchored the band's famous marathon jams.
"When Phil's happening the band's happening," Garcia once said.
Drummer Mickey Hart called him the group's intellectual who brought a classical composer's mind-set and skills to a five-chord rock 'n' roll band.
Lesh credited Garcia with teaching him to play the bass in the unorthodox lead-guitar style that he would become famous for, mixing thundering arpeggios with snippets of spontaneously composed orchestral passages.
Fellow bass player Rob Wasserman once said set Lesh's style set apart from every other bassist he knew of. While most others were content to keep time and take the occasional solo, said Wasserman, Lesh was both good enough and confident enough to lead his fellow musicians through a song's melody.
"He happens to play bass but he's more like a horn player, doing all those arpeggios - and he has that counterpoint going all the time," he said.
Lesh began his long musical odyssey as a classically trained violinist, starting with lessons in third grade. He took up the trumpet at 14, eventually earning the second chair in California's Oakland Symphony Orchestra while still in his teens.
But he had largely put both instruments aside and was driving a mail truck and working as a sound engineer for a small radio station in 1965 when Garcia recruited him to play bass in a fledgling rock band called The Warlocks.
When Lesh told Garcia he didn't play the bass, the musician asked, "Didn't you used to play violin?" When he said yes Garcia told him, "There you go, man."
Armed with a cheap four-string instrument his girlfriend bought him, Lesh sat down for a seven-hour lesson with Garcia, following the latter's advice that he tune his instrument's strings an octave lower than the four bottom strings on Garcia's guitar. Then Garcia turned him loose, allowing him to develop the spontaneous style of playing that he would embrace for the rest of his life.
Lesh and Garcia would frequently exchange leads, often spontaneously, while the band as a whole would frequently break into long experimental, jazz-influenced jams during concerts. The result was that even well-known Grateful Dead songs like "Truckin'" or "Sugar Magnolia" rarely sounded the same two performances in a row, something that would inspire loyal fans to attend show after show.
"It's always fluid, we just pretty much figure it out on the fly," Lesh said, chuckling, during a rare 2009 interview with The Associated Press. "You can't set those things in stone in the rehearsal room.
Lesh was a Berkeley native and lived in Marin at the time of his passing.