Bluegrass guitarist-singer Lester Flatt died yesterday in a Nashville hospital at the age of 64.

Mr. Flatt, who had undergone heart surgery in 1975 and other major surgery a year later, had returned to the stage of the Grand Ole Opry in March although he had been described as being near death last November after a brain hemorrhage.

Half of the Flatt and Scruggs team in the early 1960s that led bluegrass music's resurgence, Mr. Flatt described a few months ago his most cherished accomplishments: "We should be proud of being able to accomplish what we set out to do. We wanted to be in a field to ourself. We had a sound all our own."

Expressing his belief that the acoustic style of country-mountain music will not die out, he said: "There always will be somebody to carry it on. It's been good for us since it started."

Mr. Flatt's last appearance in this area was Oct. 15 at a bluegrass festival near Williamsburg. Then, as during the whole 1978 festival season, he sat on a stool while playing his famous rhythm guitar and singing his most popular songs.

These included "The Ballad of Jed Clampett," which had meant instant recongnition for him, his former partner, banjoist Earl Scruggs, and the Foggy Mountain Boys as the theme song for television's "Beverly Hillbillies." The performance last October further impressed upon the fans the steady deterioration of Mr. Flatt's health. They remembered when his shows included the "Granny" dance, performed on television by Irene Ryan.

Mr. Flatt, a native of Overton County in East Tennessee, got his first musical inspiration from his father and his first attempts in music were on the banjo. But the fine musical ear that was to be his passport to a career soon told him that the five-string instrument was not for him, and he switched to the guitar.

Like so many country musicians in the early 1930s, Mr. Flatt had to look elsewhere for money to support his family, and the cotton mills of North Carolina and Virginia were the answer. Therefore, Mr. Flatt was no youngster when he finally gave up the security and the drudgery of the mills to rely on his voice and his guitar for a living.

By the early 1940s, he and his wife, Gladys, were touring with one of the most popular bands in the Southeast, Charlie Monroe and the Kentucky Pardners. It was during this period that his distinctive voice began to be recognized, as he sang the tenor part over Monroe, the elder brother of Bill Monroe, known as the father of bluegrass.

Soon after Lester and Gladys Flatt left Charlie Monroe's band, the call came from Nashville seeking his voice and guitar for Bill Monroe's band, The Bluegrass Boys, who had made their entry into radio station WSM's Grand Ole Opry a few years earlier.

It was with Bill Monroe that Mr. Flatt found his future partner, Scruggs, with whom his name is most often linked. It was Flatt and Scruggs who were on the "Beverly Hillbillies," wrote and recorded "Foggy Mountain Breakdown," which later became the theme song for the movie "Bonnie and Clyde," recorded live at Carnegie Hall, and received a broadcast contract from Martha White Mills, a Tennessee flour firm that kept them on early-morning radio and the Opry for years. They were synonymous with the word "bluegrass" for more than 20 years.

The partnership that began in 1948 when Flatt and scruggs left Bill Monroe survived, although not without some difficulties, the surge of folk music.

Their success in those years, in fact, was considered by some to be the harbinger of their split. As Scruggs' sons became professional musicians, serious differences developed about the type of music the band should play.

The announcement surprising to few, that the partners were splitting came in February 1969. Scruggs and his sons, Randy and Gary, formed the Earl Scruggs Review.

Scruggs visited Mr. Flatt in the hospital early this year.

"Earl and I hadn't spoken in 10 years," Mr. Flatt said later. "But when I was in the hospital, Earl came to see me. It came as quite a surprise and made me feel real good."

Yesterday afternoon, Scruggs said, "He was very talented. His record speaks for itself . . . He just had a talent that people enjoyed. He was blessed with a good following. We had a lot of good memories together."

After the breakup, Mr. Flatt formed the Nashville Grass, taking with him all but one of the Foggy Mountain Boys. Such names as Paul Warren, Jake Tullock and Josh Graves, close to legendary on their particular instruments, were part of the original Nashville Grass.

In 1973, another legendary name, Curly Sechler joined the band and was still part of the Nashville Grass when its leader died.

Although counting heavily on his longtime associates, Mr. Flatt had fostered the development of new young stars, such as Marty Stuart on the mandolin and Blake Williams on the banjo. Before Stuart, there was Roland White, another young mandolin player, who had survived the extinction of the Kentucky Colonels.

This emphasis on young mandolin players was a departure from the the old Flatt and Scruggs image, mainly because of the rift that existed over the split with Bill Monroe, considered by some the epitome of bluegrass mandolinists.

After the break with Scruggs, Mr. Flatt had several successful ablums, both with the Nashville Grass and in reunion with one of his former singing partners, Mac Wiseman.

Mr. Flatt's survivors include a daughter, Brenda Green of Hendersonville, Tenn., three brothers, three sisters and two grandchildren. CAPTION: Picture, Lester Flatt, in recent photo, had been a Grand Ole Opry star since 1944.