Aaron Copland, one of the most widely respected and frequently performed serious American composers of this century, died yesterday of complications from several strokes. He was 90.

A resident of Peekskill, N.Y., Copland died at a hospital in North Tarrytown, N.Y. His death was announced by his lawyer, Ellis Freedman, who said the composer also had been suffering from respiratory problems.

Copland's work ranged from ballet to motion picture soundtracks. It included such popular compositions as "Rodeo" and "Appalachian Spring," which won a Pulitzer Prize in 1944, and such abstract works as his Third Symphony, which was called the "greatest American symphony" by his friend Serge Koussevitzky, director of the Boston Symphony Orchestra.

Although primarily known as a composer, Copland also was a critic, lecturer, author, teacher and conductor, and in a musical career that lasted more than six decades he led some of the great symphony orchestras of the world.

Copland borrowed heavily from traditional American folk tunes for much of his music, which was immensely popular with a broad segment of the public. But he also wrote intricate 12-tone pieces for a much smaller audience of musically sophisticated listeners.

He composed "A Lincoln Portrait," for orchestra and a narrator, using variations on popular American tunes of Lincoln's time and a text derived from Lincoln's writings and speeches on democracy; "Fanfare for the Common Man," a World War II-era expression of the grandeur of American ideals; and "Billy the Kid," a hymn of praise to the rawness and vitality of the American West.

He wrote the musical scores for eight movies, including "Our Town," "Of Mice and Men" and "The Heiress." In the process, he helped set new soundtrack standards for the motion picture industry.

In his soundtracks, Copland used his music to tell the audience how to feel about what was happening on the screen. In one dramatic scene in "The Heiress," the audience at a sneak preview laughed inappropriately, and studio executives came to Copland in a panic. He inserted music in the soundtrack to let the audience know the scene was tragic, not funny.

Copland composed a clarinet concerto for Benny Goodman. He wrote chamber music and vocal music, including "12 Poems of Emily Dickinson," and he wrote operas, the best known of which was "The Tender Land," the story of a Midwestern farm girl who feels confined by her Depression-era isolation and wants wider horizons.

He came of age professionally during the 1920s and 1930s, just as the radio and phonograph were gaining popularity, and those developments had a profound effect on his career.

Copland wrote in an autobiographical sketch that "an entirely new public for music had grown up around the radio and phonograph. It made no sense to ignore them and to continue writing as if they did not exist. I felt it was worth the effort to see if I couldn't say what I had to say in the simplest possible terms."

Beginning in the mid-1930s, Copland started to incorporate American folk music, ranging from cowboy songs to New England hymns, into many of his compositions. The best-known example is probably the Shaker hymn " 'Tis a Gift to Be Simple," which provides the basis for the climactic section of "Appalachian Spring."

Critics said the device enabled Copland to achieve a "homespun musical idiom" and a "mellow warmth and lyrical quality" that gave his music wide appeal.

In the latter decades of his career, Copland all but stopped composing, but he spent more time conducting. "After 50 years composing, you tend to slow up a bit," he said when he was 72. "The urge is still there, but it's not as strong as it was 30 years ago."

He traveled all over the world as a conductor, and he was still leading symphony orchestras well into his eighties. It helped keep him young, he said. "You have an athletic workout -- you come off the stage sweating like a pig."

He was in Washington dozens of times. He conducted outdoor concerts at the Capitol and received tributes at the Kennedy Center, where in 1980 President Carter called him "America's foremost composer."

The nation's leaders had not always held him in such esteem. "A Lincoln Portrait" was abruptly withdrawn from the program for President Eisenhower's inaugural concert in January 1953 after Rep. Fred E. Busbey (R-Ill.) complained that Copland had associated with communist front organizations.

Four months later, Copland was called to testify before the Senate investigations subcommittee headed by Sen. Joseph R. McCarthy (R-Wis.) to explain any connections with communist organizations.

"I never have been and am not now a communist," he told the committee. During the 1940s, he said, he had inadvertently permitted the use of his name by certain organizations without knowing they had communist connections.

But Copland bore no lasting scars from that experience. "That was so long ago. I took it as one of the facts of life. And things in life have gone so well for me," he said when asked about it 30 years later.

Aaron Copland was born in Brooklyn on Nov. 14, 1900, the youngest of five children of a department store owner. An older sister began teaching him to play the piano when he was 11, and two years later he persuaded his parents to permit him to begin taking lessons.

When he was about 15, he decided he wanted to be a composer, and he began studying harmony and composition. In 1921 he went to France as the first student to enroll at the new American Conservatory at Fontainebleau. While studying composition there, he visited a harmony class being conducted by Nadia Boulanger in Paris, and he was so impressed by her teaching that he decided to move to Paris and study under her.

He did so for the next three years. Boulanger introduced Copland to Serge Koussevitzky, who years later, after his appointment as head of the Boston Symphony Orchestra, became one of Copland's most enthusiastic admirers.

Copland returned to the United States in 1924 after Boulanger asked him to compose something for her to play during an American tour as an organ soloist with several orchestras. He produced "Symphony for Organ and Orchestra," which was given its premiere by the New York Symphony, with Boulanger playing organ, on Jan. 11, 1925.

During the summers of 1926, 1927 and 1929, he returned to Europe to keep abreast of the latest developments in music, and he helped sponsor an important series of "new music" concerts in New York in the late 1920s and early 1930s. In this period, Copland also lectured at the New School for Social Research, and his lectures and articles were published as two books, both of which were well received critically and translated into several languages.

His musical compositions during this period included chamber music, a short symphony and piano concertos, described by critics as "complex and uncompromisingly austere," characterized by "nervous, irregular rhythms and angular, jagged nonmelodic thematic material." Most were difficult to perform and difficult for listeners to understand.

Not until the mid-1930s did Copland begin to write for general audiences. One of his first such compositions was "El Salon Mexico," a 1936 orchestral piece based on traditional Mexican melodies. It was followed during the next 10 years by a series of works based on American folk music.

He wrote "A Lincoln Portrait" in 1942 for orchestra leader Andre Kostelanetz. Carl Sandburg, the American poet and Lincoln biographer, was the first narrator. Other notable narrators have included Adlai Stevenson and Coretta Scott King. In October 1944, "Appalachian Spring" had its world premiere at the Library of Congress.

From 1940 until 1965, Copland was on the summer school faculty of the Berkshire Music Center, established by his friend Koussevitzky, in western Massachusetts, and he served three one-year appointments on the faculty at Harvard in 1933, 1944 and 1952.

Beginning with his Piano Quartet in 1950, he turned once again to compositions with a European flavor, but this time his works seemed to have more of a personal element than his earlier pieces.

As he grew older, Copland began to look the part of musical patriarch, his features becoming craggy and forbidding. But his manners were gentle and easygoing, and he made a practice of giving advice and counsel to struggling young composers, including Leonard Bernstein, whom he had known at Harvard. Copland was also adept at finding sources of finances -- patrons and foundations -- for such young artists.

Copland never married. He leaves no survivors.