Charles Mingus, 56, one of the first jazz musicians to use the bass as a solo instrument and a major modern jazz composer, died Friday in Cuernavaca, Mexico. He had been suffering since 1977 from amyotrophic lateral sclerosis, a disorder of the nervous system sometimes called Lou Gehrig's disease.

He had attended the White House Jazz Festival in July in a wheelchair and was embraced by President Carter in one of the day's most affecting moments.

According to a childhood friend and subsequent colleague, saxophonist Buddy Collette, Mr. Mingus had gone to Mexico about five months ago seeking medical help. Collette said the body was cremated Saturday.

In the last decade Mr. Mingus had been in semiretirement for health reasons. His recent work only occasionally bristled with the turbulent emotional power and lyrical delicacy that had marked it since his most active and creative period in the late 1950s and early 1960s.

In the mid-1970s, he put together a group that met with mixed critical reaction. Nevertheless, one of the band's records, "Three or Four Shades of Blue," released in 1977, sold more than 50,000 copies, a large figure for a jazz record.

Near the end of his life, Mr. Mingus was working on an album in an unlikely combination with pop singer Joni Mitchell. The record reportedly will feature Mitchell lyrics set to Mr. Mingus' music.

The well-known Mingus temper -- the one that once led him to punch a drunk who was shouting racial insults in a nightclub -- was in evidence almost to the end. It flared during the musician's last performance in Washington in September 1977, when he discovered that his sidemen had left their music in New York.

He stopped the performance and trudged off the bandstand at the Cellar Door, threatening to cancel the engagement. But Sue Graham, who was his manager as well as his wife, persuaded him to stay.

Mr. Mingus' outspoken opinions extended to large social and political issues. He would lecture nightclub audiences on current affairs. He wrote poetry with social themes and gave his works such titles as "Meditations on Integration," "Better Git In Your Soul," "Oh, Lord, Don't Let Them Drop That Atomic Bomb on Me" and "Fables of Faubus."

Mr. Mingus also believed in unearthing the racial roots of his music.

Unlike many modern jazzmen, he acknowledged the influence of Duke Ellington, Jelly Roll Morton and Art Tatum.

He also was a master of the put-on. "The Watts riot was a fake," he said in an interview. "I know everybody in Watts (he grew up there). A truck drove up with some strangers and they did the burning."

Some of his pieces were titled "Don't Be Afraid, the Clown's Afraid Too," "All the Things I Could Be If I Were Sigmund Freud's Mother" and "The Shoes of the Fisherman's Wife are Some Jive Ass Slippers."

But his music was always serious, sometimes reaching a brooding intensity of Wagnerian scope. Mr. Mingus always seemed to be trying to push his bass and his compositions beyond the accepted limits of form and technical restrictions.

In his last appearance at the Cellar Door, Mr. Mingus weighed 262 pounds, down from 312 pounds. He stood 5-feet-9. Seated, he looked like a coiled Buddha.

"For five years I didn't want to think about music," he said. "I played the piano a little. I didn't touch the bass. Just this year I'm getting my energy to start writing again."

The musician said he had been through his most trying years. They included eviction in 1966 from his Lower Manhattan loft for nonpayment of rent and later rebuffs of his attempts to start a music school where children could study jazz.

He published an autobiography in 1971, "Beneath the Underdog," an unbridled account of his sexual excesses and associations with pimps and whores with little focus on his music.

Born in Nogales, Ariz., Mr. Mingus grew up in Los Angeles. Before organizing his own group, he performed with a "Who's Who" of jazz: Louis Armstrong, Kid Ory, Lionel Hampton, Red Norvo, Charlie Parker, Stan Getz and Art Tatum.

Beginning in the late 1950s, his ensembles served as a forum for experimental jazz. He even called his group at one time the Charles Mingus Jazz Workshop. Through his band passed such musicians as Eric Dolphy, Rahsaan Roland Kirk, Jackie McLean and Jaki Byard.

Mr. Mingus' music for his Jazz Workshop groups included extended works, sometimes requiring medium sized orchestras. His style featured the thumping, call-and-response rhythms of the Pentecostal and Holiness churches he attended as a child. It also was characterized by a full ensemble sound, sometimes resembling Ellington's.

He required his soloists to tailor their styles somewhat to his. But in most cases this worked to their benefit. Mr. Mingus often was able to elicit performances from many of his sidemen that were superior to their customary efforts.

He had a lasting impact as a teacher. Critic Nat Hentoff once wrote: "Mingus hovers over his men like a brooding Zeus making up the final score card for eternity."

Mr. Mingus is survived by his wife, three sons, two daughters and two sisters.