An esteemed author of novels, short stories, essays and criticism, Peruvian Mario Vargas Llosa is one of Latin America's major writers, in the company of Gabriel Garcia Marquez, Octavio Paz, Carlos Fuentes and the late Julio Cortazar. Turning to politics as a moral imperative and from a sense of adventure, Mr. Vargas Llosa ran for the presidency of his troubled country in 1990. He was defeated by Alberto Fujimori, who in 1992 suspended democracy, turning Peru once again into a dictatorship. Now an expatriate living in London, Mr. Vargas Llosa is a vocal critic of the Fujimori regime.

In this masterful autobiography, Mr. Vargas Llosa tells two stories in alternating chapters: first, that of his childhood and youth, from his birth in Arequipa in 1936 to his departure in 1958 for an eight-year sojourn in France, and, second, that of the failed 1989-1990 presidential campaign.

He examines his country's terrible deterioration over the past 30 years. "A country can always be worse off. Underdevelopment is bottomless," he declares with hard-won, justifiable pessimism. The Peru into which he was born was a society divided into economic, ethnic and racial hierarchies characterized by resentment and envy of those on top and contempt of those below. Still, it was far from the violent, marginalized society that it has become today.

In the brilliant first chapter, "The Man Who Was My Papa," Mr. Vargas Llosa relates how his father, whom he had never known and whom he had been told was dead, suddenly appeared in his life when he was 10 years old and, without so much as a warning, took him and his mother away with him. Mario's carefree youth among his mother's extended family in sunny Cochabamba, Bolivia, and Piura, Peru, was exchanged for a grim existence under his father's abusive tyranny in gray Lima.

Mr. Vargas Llosa describes how his love of literature and its imaginative life was nourished as a secret resistance against circumstances he found unbearable, first with his father and then as a boarding student in a military academy.

In successive chapters, we observe his life change with dizzying rapidity. At 15, he became a journalist in search of sensational stories; the next year he lived with a beloved maternal uncle in the Piura of his boyhood. It was here that he enjoyed his first literary success: He wrote a prize-winning historical play.

He returned to Lima, studying literature and law at the University of San Marcos, where he fell under the sway of history and politics, and became a leftist intellectual and the valued research assistant of an eminent historian.

Readers of Mr. Vargas Llosa's fiction will recognize the inspiration for "The Time of the Hero" and "Conversation in the Cathedral." At 19, while working in a radio station, Mr. Vargas Llosa fell in love with his 32-year-old "Aunt" Julia -- actually the sister of his uncle's wife. Despite their family's resistance and the furious threats of Mr. Vargas Llosa's father, they eloped, an adventure hilariously recounted in "Aunt Julia and the Scriptwriter."

Meanwhile, he dreamed of going to Paris, where he hoped to transform himself into a true writer. This dream was first realized in January 1958, when he won a short-story contest. He won a two-week trip to Paris, and later that year he received a fellowship to study abroad.

Since Mr. Vargas Llosa alternates the story of his coming of age with his account of the presidential campaign, the autobiography reads like a serial with a double plot, each one temporarily departed from only to be continued. By approaching his life from both ends, the distant past and the recent past, he complicates and enriches our understanding of his character.

"My vocation is that of a cosmopolitan and an expatriate who has always detested nationalism, which strikes me as one of the human aberrations which has made the most blood flow," he proclaims. Such sentiments are unusual in a presidential candidate. Had he won the election, Mr. Vargas Llosa's program might have resembled that of another cosmopolitan writer-president, Vaclav Havel of the Czech Republic, who also ran on a platform committed to democratic freedoms, economic development through a market economy, and an end to the corrupt state socialism of previous regimes.

The candidate of a coalition of political parties, the Democratic Front, Mr. Vargas Llosa also argued for the arming of civilian militias as the most effective way to defeat the anarchic terrorism of the Shining Path guerrillas on the left and the state-condoned terrorism of the armed forces.

In these pages, he analyzes campaign strategies, judges his supporters and opponents, and remorselessly admits his miscalculations and failures. While the detail is exhaustive, the rewards are Mr. Vargas Llosa's thoughtful revelations and stunning evocations of a difficult campaign carried out in a country of majestic geographical extremes whose infrastructure lies in tatters.

With the surprising ascent of a dark-horse candidate in Mr. Fujimori, an obscure agricultural engineer, who Mr. Vargas Llosa assumed -- correctly, it seems -- was a tool of the powerful military, the campaign became mired in issues of religion and racism that Mr. Vargas Llosa had wanted to avoid (Mr. Fujimori, an evangelical Christian, is the son of Japanese immigrants).

Mr. Vargas Llosa found himself "trapped in a spider web of misunderstandings," created both by opponents and supporters. Realizing that he was bound to lose, he was relieved when at last the campaign came to an end, and he could return to his

independent life as a writer.

This impressive autobiography comes to an end long before the lives of the prize-winning aspiring writer and the 54-year-old presidential candidate converge. Mr. Vargas Llosa leaves many questions unanswered; among them: How, in his years abroad, did he become the writer he had longed to be? How were the leftist politics of his youth transformed to his support of the liberal democratic ideal of a civil society and a free-market economy?

How did his romantic marriage with his aunt Julia founder and come to an end, and how did he fall in love with and marry his cousin Patricia, daughter of his beloved Piuran uncle? Given his relationship with his father, what kind of father was he to his two sons and daughter? Like all good cliffhangers, this fascinating .. memoir leaves us eagerly anticipating a sequel.

Ms. Whitehouse is a writer who lives in New York.

Title: "A Fish in the Water: A Memoir"

Author: Mario Vargas Llosa; translated from the Spanish by Helen Lane

Publisher: Farrar, Straus and Giroux

-! Length, price: 532 pages, $25