Her son, Theo Downes-Le Guin, said the cause was not immediately known.
While Ms. Le Guin occasionally ventured into realistic fiction, she aimed to avoid the standard fare of contemporary literature, books that she once derided as "fiction about dysfunctional urban middle-class people written in the present tense."
Instead, she populated her novels with richly imagined worlds that drew less from recent science fiction than from ancient mythology or Taoism, the Eastern philosophy that emphasizes acceptance and change. Ms. Le Guin once translated the ancient "Tao Te Ching," publishing her take on the Taoist classic amid novels, stories and books of essays and poetry that made her one of the most beloved writers in American literature.
She received an honorary National Book Award in 2014 for distinguished contribution to American letters; became a Pulitzer Prize finalist with her 1996 collection "Unlocking the Air and Other Stories"; and in recent years was rumored to be a candidate for the Nobel Prize in literature. Last year, the British gambling site Ladbrokes put her odds of winning the honor at 33 to 1.
Still, Ms. Le Guin's fantastical writing style made her something of a literary outsider — a role that she embraced in later years, decrying profit-minded publishers who market writers "like deodorant, and tell us what to publish, what to write."
One of her most acclaimed novels, "The Left Hand of Darkness" (1969), was initially published not as a work of hardcover literature but as a 95-cent mass-market paperback.
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The book received the Nebula and Hugo awards, two of science fiction's highest honors, but Ms. Le Guin saw the novel — and all her books that followed — as reaching beyond the genre. Part of a series known as the Hainish Cycle, which included her 1974 book "The Dispossessed," it centered on a planet of androgynous, humanlike beings.
"The form of the book is a mosaic of primary sources, an interstellar ethnographer's notebook, ranging from matter-of-fact journal entries to fragments of alien myth," novelist John Wray wrote in the Paris Review.
The novel was cited by literary critic Harold Bloom in "The Western Canon," his overview of classic literature, and paved the way for Ms. Le Guin's broader acceptance, which began in full with her Earthsea series for young adults.
The books, centering on a young wizard named Ged who comes to terms with sex, death and other rites of adulthood, have sold more than 1 million copies and been translated into more than a dozen languages. Their third installment, "The Farthest Shore," received the National Book Award for children's literature.
"Though marketed as young-adult novels, 'A Wizard of Earthsea' (1968), 'The Tombs of Atuan' (1971) and 'The Farthest Shore' (1972) are as deeply imagined, as finely wrought, as grown-up as any fiction of our time," Washington Post book critic Michael Dirda wrote in 1990. "They deserve that highest of all accolades: Everyone should read them."
Ursula Kroeber was born in Berkeley, Calif., on Oct. 21, 1929. Both her parents were anthropologists who studied American Indians in California; her mother was the author of "Ishi in Two Worlds," a popular 1961 volume that was subtitled "a biography of the last wild Indian in North America."
She said that her father's relating of Indian legends provided her introduction to fantasy worlds, prompting her to explore similarly folkloric books by Norwegian and Irish authors before her literary epiphany, at age 12, with Irish fantasy writer Lord Dunsany's "A Dreamer's Tales."
"What I hadn't realized, I guess, is that people were still making up myths," she later wrote in an essay. "One made up stories oneself, of course; but here was a grown-up doing it, for grown-ups, without a single apology to common sense, without an explanation, just dropping us straight into the Inner Lands. I had discovered my native country."
She studied Renaissance literature at Radcliffe College, graduating in 1951, and one year later received a master's degree from Columbia University with a thesis on representations of death in the French poetry of Pierre de Ronsard.
While traveling to Paris on a Fulbright fellowship, she met historian Charles Le Guin and decided to set aside her doctoral studies. They married in 1953. In addition to her husband and son, both of Portland, survivors include two daughters, Caroline Le Guin of Oregon City, Ore., and Elisabeth Le Guin of Santa Ana, Calif.; two brothers; and four grandchildren.
Ms. Le Guin wrote poetry and short stories, many of them realist in style, before returning to science fiction in the 1960s, inspired in part by the stories of Paul M.A. Linebarger, who wrote under the pseudonym Cordwainer Smith.
Her books often anticipated other, more-popular fantasy fare, New York Times journalist David Streitfeld observed in 2016: "The Word for World Is Forest" (1976), about humans invading a planet of peaceful, nature-loving aliens, seemed an inspiration for James Cameron's blockbuster movie "Avatar"; "Planet of Exile" (1966), where the seasons last 15 years and creatures attack from the frigid north, pointed toward "Game of Thrones."
Ms. Le Guin went on to win several lifetime-achievement awards for science fiction, including the Grand Master award from the Science Fiction & Fantasy Writers of America, and in recent years stretched far beyond the genre in books such as "Lavinia" (2008), which made a minor character from Virgil's "Aeneid" the star of her own story.
In large part, the novel was a continuation of a lifelong interest of Ms. Le Guin's, a form of feminism that she preferred to describe as humanism.
Delivering the 1983 commencement address at Mills College in Oakland, Calif., she described a future where young women attained the same sort of independence achieved by many of her characters.
"Why should a free woman with a college education either fight Machoman or serve him?" she said, in what rhetoric scholars later listed as one of the top 100 political speeches of the 20th century. "Why should she live her life on his terms? . . . I hope you live without the need to dominate, and without the need to be dominated."
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