With his rugged masculinity overlaying a simmering emotional vulnerability, Heathcliff Ledger was among the finest young actors of his generation. A daring chameleon in his choice of roles, he played Casanova, Bob Dylan, and a gay cowboy in “Brokeback Mountain” (2005), for which he was nominated for an Academy Award.

He was the latest in a line of Australian actors to take Hollywood, including Nicole Kidman, Cate Blanchett, and Mel Gibson, who helped Ledger achieve early stardom when the two starred together in “The Patriot” (2000). He recently finished filming “Dark Knight,” the latest movie in the “Batman” series, slated for release later this year.

In New York, Ledger became an icon of the glamour set, settling in downtown Brooklyn with his girlfriend Michelle Williams — who played his wife in “Brokeback Mountain.” The tall, tattooed Ledger became a familiar sight, tooling around Boerum Hill on his skateboard. The pair had a daughter, Matilda, but they split late last summer, and Ledger was reportedly living in a SoHo apartment.

Born April 4, 1979, in Perth, Australia, Ledger caught the acting bug early, starring in a production of “Peter Pan.” Although tabbed as a promising field hockey player, he stuck with acting and in high school led his high school drama team to a national dance championship. He dropped out of school after tenth grade and found work on several Australian television shows. In 1997, former teen idol David Cassidy cast him as Conor in “Roar,” a Fox network series about Highland Celts in Roman times that critics compared to the film “Braveheart” (1995).

Ledger later moved to Los Angeles, where he was cast in what became his breakthrough role in the Petruchio role opposite Julia Stiles as Kate in “10 Things I Hate About You” (1999), a modern high school take on “The Taming of the Shrew.” Also in 1999, Ledger was nominated for an American Film Institute award for best actor for his role in “Two Hands,” an Australian gangster film screened at the Sundance Film Festival.

After coming to prominence in “10 Things,” Ledger was offered numerous “pretty boy” roles, but turned them down, borrowing money from his agent and infuriating his parents. In 2001, he romped through the 14th century in “A Knight’s Tale” and then had a smaller role as a suicidal prison guard in “Monster’s Ball.” His other films included “The Four Feathers” (2002), “Ned Kelly” (2003), and “The Brothers Grimm” (2005).

In recent months, Ledger had been sighted in Perth escorting Australian model Gemma Ward, and earlier reports linked him with other starlets of comparable magnitude.

Michael Caine, Ledger’s co-star in the forthcoming “Dark Knight,” told Britain’s Press Association that Ledger’s performance as the Joker is “very, very scary” — so striking that the normally unflappable Mr. Caine, who plays Batman’s butler, Alfred, said he forgot his own lines.