Actor William Holden, 63, who overcame the handicap of extreme good looks and went on to play tough, cynical and enormously appealing versions of the All-American male, was found dead yesterday in his apartment in Santa Monica, Calif.

Mr. Holden's body was found next to his bed by the building manager. He had gone to the Santa Monica residence from his permanent home in Palm Springs last week on a business trip, according to a member of his household. An official of the coroner's office said the death, which may have occurred a week ago, "apparently was due to natural causes." An autopsy was scheduled for today.

There were reports last year that Mr. Holden had cancer.

Mr. Holden's career spanned more than 40 years. It began in 1939 with the title role in "Golden Boy," the story of a kid from the slums who wanted to be a violinist but who was driven by hard times to be a prize fighter. It ran through such classics as "Stalag 17," "Bridge on the River Kwai" and "Network" and some of the biggest box-office smashes of the post-war era, including "Love Is a Many-Splendored Thing," "The World of Suzie Wong" and "The Towering Inferno."

In all, Mr. Holden starred in 70 films, including "S.O.B.," a satire about Hollywood released earlier this year. The striking baritone in which he spoke was evident in all of them and his splendid physique was on display in many of them. Otherwise, his vehicles were as diverse as those of most other stars of his time. They ranged from comedy to action to melodrama to the edge of tragedy.

They brought Mr. Holden an Academy Award (for Best Actor of the Year in 1953 for "Stalag 17") and nominations for best actor for "Sunset Boulevard" (1950) and "Network" (1976). They won him the respect and friendship of such fellow-actors as Ronald Reagan, Glenn Ford and Gregory Peck and of directors such as Billy Wilder.

He was the best man at Reagan's wedding to the former Nancy Davis in 1952. On hearing of Mr. Holden's death, the president said, "I have a great feeling of grief. We were close friends for many years.... Our friendship never waned," according to a White House spokesman.

Off the screen, Mr. Holden was an avid nature conservationist. He helped establish the Mount Kenya Safari Club in Kenya and at one time maintained a residence there. He also traveled widely elsewhere in Africa and in Asia. At one time his principal home was in Switzerland and he also had an apartment in Hong Kong. He contributed to anthropological studies in New Guinea and other remote reaches of the world.

"My blueprint is to make one very important picture a year, one that is artistically satisfying to me but is successful at the box office," he said in an interview. "I'd like to spend six or seven months on my vocation and five or six months on my avocations."

But that was in 1960, when he already was one of the most popular personalities in the film industry. After "Golden Boy," there were years and years when he seemed to play only the handsome lad next door. In 1940, he was in "Invisible Stripes," "Those Were the Days," "Our Town" and "Arizona." In 1941, it was "Texas" and "The Remarkable Andrew."

During World War II, he was a lieutenant in the Army Air Forces. When he went back to Hollywood, he was unemployed for seven months. He then appeared in such films as "Blaze of Noon" and "Dear Ruth." Mr. Holden referred to these as his "smiling Jim" portrayals.

His first big break came in 1950 in "Sunset Boulevard," in which he played Joe Gillis, a down-and-out writer who becomes the gigolo of an aging film star played by Gloria Swanson. It was the first of the tough-but-nice, soft-but-cynical roles that became a Holden trademark.

"I got into the rut of playing all kinds of nice guy, meaningless roles in meaningless movies, in which I found neither interest nor enjoyment," Mr. Holden said after "Sunset Boulevard." "By 1950, I had appeared in 11 movies, but, for me, they added up to one big static blur.... I was waiting and hoping for, and struggling toward, that particular movie that would show just how individual I could be, as actor and as man, and finally I found it."

In 1953, with the release of "Stalag 17," he became a major star. Directed by Billy Wilder, the film was a story of allied airmen in a German prison camp during World War II. Mr. Holden played Sefton, a kind of prototype for the anti-hero who has since become a commonplace.

Sefton was not much for camaraderie or patriotic platitudes. He was for wheeling and dealing and an easier lot for himself. He made alcohol out of prunes and sold it to other prisoners for cigarettes. He traded the cigarettes to German guards for cameras and other luxuries, including a telescope. For a price of two cigarettes, other prisoners could use the telescope to look into the Russian women's prisoner compound for 20 seconds. Sefton's mates suspected him of treachery. In the end, he unmasks the real spies among them and takes their place in a great escape attempt.

Mr. Holden played a similar role in "Bridge on the River Kwai." Not for him was the sense of order and duty personified by Alec Guinness. The Holden character escapes and returns to help blow up the bridge that Guinness has created.

Other memorable Holden roles in the 1950s were in "The Bridges of Toko-Ri," which also starred Grace Kelly, "Escape From Fort Bravo," which includes one of the most realistic Indian attacks ever filmed, "Love Is a Many-Splendored Thing" and "Picnic."

By this time, Mr. Holden had become financially secure. He is said to have been the first major star to take deferred payments for films in order to reduce tax liability. He did this with "Bridge of the River Kwai." In 1959, he and John Wayne each were paid $750,000 in salary and 20 percent of the gross receipts for "The Horse Soldiers."

In 1960, he made "The World of Suzie Wong," an engaging story about a prostitute in Hong Kong.

By then, Mr. Holden had become a pillar of respectability in Hollywood. He was a vice president of the Screen Actors Guild and was known as a union official who would negotiate as hard for others as he did for himself. He stayed away from the nightlife for which Tinsel Town is famous.

During the 1960s, however, he made fewer movies and spent more time on his other projects. In 1963, he separated from his wife, actress Brenda Marshall, and in 1970 they were divorced. There were stories that he was drinking heavily. In fact, he always enjoyed a drink. Accoring to well-established legend, one of his favorite expressions on the set was, "Warm up the ice cubes."

"Over the years, I try to keep a standard, but nobody bats a thousand," Mr. Holden said in an interview in 1967. "I haven't made that many pictures recently. I have too many other interests. The chief ones are game conservation and exploration in Africa."

But some of the films that Mr. Holden made during the last decade of his life were as good as any he made in his career. The best, perhaps, was "Network," a bitter comedy about competition in television. Mr. Holden played a news executive whose principles remain more of less intact, but whose life, both professional and private, is falling apart.

William Franklin Beedle Jr. was born on April 17, 1918, in O'Fallon, Ill. His father, William Franklin Sr., was a prosperous industrial chemist. His mother was a school teacher. The future star grew up in South Pasadena, Calif. By all accounts, he was a model boy -- he sang in the church choir, did his chores, and worked hard at school -- and he had a model boyhood. His father taught him tumbling. As he grew older, he often sought danger. When he was given a motorcycle, he rode it while standing on the seat. He once walked the outer ledge of a bridge known as the site of suicides, ignoring the 190-foot drop at his feet.

At Pasadena Junior College, he became interested in acting. In 1937, he played Marie Curie's 80-year-old grandfather in a play called "Manya," which was put on by a local theater. Milton Lewis, a scout from Paramount Studios, spotted him and signed him at $50 a week.

Mr. Holden and Miss Marshall, whom he married in 1941, had two sons. Peter Westfield and Scott Porter.