Phil Lesh, whose expansive approach to the bass as a charter member of the Grateful Dead made him one of the first performers on that instrument in a rock band to play a lead role rather than a supporting one, died on Friday. He was 84.

His death was announced on his Instagram account. No further information was provided.

In addition to providing explorative bass work, Mr. Lesh sang high harmonies for the band and provided the occasional lead vocal. He also co-wrote some of the band’s most noteworthy songs, including ones that inspired adventurous jams, like “St. Stephen” and “Dark Star,” as well as more conventional pieces, like “Cumberland Blues,” “Truckin’” and “Box of Rain.”

Key to the dynamic of The Dead was the way Mr. Lesh used the bass to provide ever-shifting counterpoints to the dancing leads of the lead guitarist Jerry Garcia, the curt riffs of the rhythm guitarist Bob Weir, the bold rhythms of the drummers Mickey Hart and Bill Kreutzmann, and, in the band’s first eight years, the warm organ work of Ron McKernan, known as Pigpen.

A source of particular excitement was the relationship between Mr. Lesh’s instrument and Mr. Garcia’s. At times they mirrored each other. At other times they contrasted, in the process widening the music’s melodic nuances while helping to create the kind of variety and tension that allowed the band to improvise at length without losing the listener.