Allan Holdsworth, a self-taught guitarist whose protean, virtuosic style was a source of amazement even to his more famous peers, died on Saturday at his home in Vista, Calif. He was 70.

The cause was a heart attack, said Leonardo Pavkovic, Mr. Holdsworth’s former manager.

Mr. Holdsworth forged a relentlessly exploratory approach to harmony, which he brought to bear on both the guitar and the SynthAxe, a guitarlike synthesizer that allowed him added control over his tone and flow. He had his own vocabulary of unorthodox chords, often involving far reaches across the fretboard. As a soloist, he executed lightning-fast melodies with remarkable fluidity.

Reviewing a performance by Mr. Holdsworth in 1983, The New York Times’s Jon Pareles wrote: “He pours out notes in a liquid rush without slurring a single one. His sense of harmony reveals itself in daring melodic extrapolations and in chords that are complex and impressionistic yet as transparent as folk music.”

Mr. Holdsworth played in a number of seminal ensembles throughout the 1970s, including the Tony Williams Lifetime, Soft Machine and U.K. But he never seemed to arrive at any band’s moment of peak popularity, or to stick around long enough to accrue a major following.