Vladimir Nabokov, the Russian-born novelist who became one of the most acclaimed and original stylists of contemporary English-language literature, died in Montreaux, Switzerland, of an unidentified virus ailment, it was announced yesterday. He was 78.

"He had been very sick the last year and a half," said Vera Slonim Nabokov at the Palace Hotel near Geneva, where she and her husband had lived in a suite for more than a decade. "The cause of death is uncertain, it was an infection, a virus, but it hasn't been identified."

Born in St. Petersburg, now Leningrad, in the waning years of the Tsarist regime, Nabokov was best-known as the author of "Lolita," a witty, ironic tale of sexual obsession that traced a middle-aged intellectual's incredible journey across America. The book caused a scandal when first published in 1955, but eventually became a best-seller, bringing Nabokov to the attention of a mass public that had overlook his previous works in Russian and English.

Though Nabokov acquired U.S. citizenship in 1945 and taught for many years at Harvard, Cornell and other American universities, he never lost his cosmopolitan outlook. "I am an American writer," he once told an interviewer, "born in Russia and educated in England, where I studied French literature before spending 15 years in Germany."

A learned cultured man with a variety of interests - two species of butterflies and one of moths bear his name, reflecting a life-long interest in entomology - Nabokov was descended from a wealthy and powerful family. His grandfather was Minister of Justice under Tsars Alexander II and III, and his father, a prominent lawyer, was one of the founders of the "Kadets," a liberal, bourgeois party opposed both to Tsarist absolutism and the Bolsheviks.

The comfortable, idyllic existence of Nabokov's aristocratic early years, recounted in "Speak, Memory," an autobiography hailed by critics as one of the finest memoirs ever written, was shattered in 1917 by the Russian Revolution. The $2 million that had been left to Nabokov by an uncle was lost, and he and his family were forced to flee to England. There he entered Trinity College at Cambridge, graduated in 1922.

As a precocious teen-ager, Nabokov had already written and had published some of his poems, but his literary career began in earnest when he moved to Berlin in the early '20s and became active in the Russian emigre community there. Over the next 15 years he wrote nine novels, all of them later translated into English.

These works, however, did not earn enough to support Nabokov and his wife Vera, a Russian Jew whom he married in 1925, so he had to supplement his income with other activities. He taught tennis and boxing, created crossword puzzles for Russian emigre publications, and translated scientific texts and literary works such as "Alice in Wonderland" and the plays of Shakespeare from English to Russian.

Translation, from Russian to English and vice versa, was to remain one of Nabokov's major interests until the end of his life. In addition to deft, highly personalized translations of Pushkin's "Yevgeny Onegin" and "The Song of Igor's Campaign," a 12th Century Old Russian epic, Nabokov, often working with his only son, Dmitri, an opera singer and sports car racer, translated into English most of his own early novels, plays, poems, essays and criticism.

The English-language phase of Nabokov's career begin, in fact, when he undertook the translation of his novel "Despair" in 1936 after, as he was to say later, "a rather grumpy Englishman" had refused to, "saying he disapproved of the book." Three years later, Nabokov accepted a post as lecturer in Slavic languages at Stanford University and made his commitment to the English language - and America - permanent.

Nabokov at first feared that his prolific output would suffer from the change of environment. "It had taken me some 40 years to invent Russia and Western Europe," he said years later, "and now I was faced with the task of inventing America."

But "The Real Life of Sebastian Knight," published in 1941, dispelled any doubts about Nabokov's ability to create in English. It was not a major work, but it did make clear that his English novels would have the same strengths that made his Russian writing so memorable: an exquisite sense of irony and an unparalleled verbal facility.

Nabokov's love of puzzles, jokes and hoaxes of any sort was particularly prominent in this novel. "Sebastian Knight" is the story of a man who loses his own identity trying to write the fictional biography of his brother and in addition to its intricate plot, the book is full of puns. Knight, for example, dies in a French village called St. Damier, the French word for "chessboard" - a place where Nabokov spent much of his free time.

Later works, such as "Pnin," the brilliant, comic story of a Russian emigre professor adrift in American academia, and the exceedingly complex "Pale Fire," had a specifically American context, but also abounded in bi- and even trilingual wordplay and allusions. In part because of this, Nabokov came to be recognized in many quarters as the most agile and clever verbal swordsman since James Joyce.

None of these works, however, earned Nabokov as much popularity, notoriety or money as "Lolita," the novel that gave a new and sexually charged meaning to the word "nymphet." Twice made into a movie, "Lolita" turned Nabokov into a celebrity and gave him the affluence and comfort that had been absent from his life since childhood.

And in Humbert Humbert, Nabokov created one of the more memborable fictional characters of recent times, Humbert's obsession with the preteen Lolita was the stuff of both tragedy and comedy, and Nabokov often said "Lolita" was his favorite work. It was also the only one of his English novels that he translated into Russian himself.

Freed from financial worries by the success of "Lolita," Nabokov gave up teaching and moved to Switzerland - because, he said, it had an "exquisite postal service, no bothersome demonstrations, also butterflies and fabulous sunsets." He continued to writer, working at a lectern in his room overlooking Lake Geneva, writing in longhand on index cards, and had in recent years published two fine novels, "Ada" and Look at the Harlequins:" and a collection of short stories, "Tyrants Destroyed."

Despite his residence abroad, Nabokov remained interested in the U.S. Six months ago, he told an interviewer that "America is the only country where I feel emotionally and mentally at home," adding that "I pay U.S. income taxes on every cent I earn at home and abroadk."

His only regret, he said, was not having won the Nobel Prize for Literature.

No word was immediately available on the existence of any works-in-progress. Nor, according to Mrs. Nabokov, have any funeral arrangements yet been made.