One of the most prestigious bastions of male chauvinism fell today with the election of writer Marguerite Yourcenar to the French Academy, the 345-year-old overseer of the purity of the French language.

Yourcenar, 76, is a Maine resident and has been a naturalized U.S. citizen for more than 30 years. The French government recently restored her French citizenship in record time so that, at the urging of President Valery Giscard d'Estaing, she could run for a seat as one of the academy's 40 "immortals."

Widely known for her best-selling novel "Hadrian's Memoirs," an imaginary diary of the Roman emperior, she is also a prolific poet, playwright, essayist, critic and a translator into French of ancient Greek poetry and of American works, including Negro spirituals.

Defying most of the traditions of the academy, she refused to campaign for the seat by making the traditional visits to the incumbent members. She made herself unavailable for coment by timimg a Caribbean cruise to coincide with today's election, which she won by 20 votes to 12 over Jean Dorst, the head of the Museum of Natural History in Paris.

Giscard sent her a telegram from Jordan, his current stop on a Middle East tour, saying: "The president of the republic, whose function makes him the protector of the French Academy, and who is personally a great admirer of your work, addresses you his warm and deferential congratulations upon your brilliant election, which consecrates the eminent place of women in French literature."

There is much doubt that Yourcenar, who has lived since 1945 in a clapboard house with a white picket fence on Mount Desert Island off Maine, will actually take much part in the work of the academy.

Asie from its rituals of reverence for French literature, the academy's essential work takes place in its meetings every thursday to revise the official dictionary of the French language. It normally takes about 50 years for each review of the language from A to Z, a pace that insures against official acceptance of fad words and usages.

Down to current times, the academy has spurned or been spurned by many of France's most eminent men of letters -- excluding from "immortality" Andre Gide, Andre Malraux, Albert Camus and Jean-Paul Sartre. Academy members, many of whom have long since been forgotten, are known as immortals because of the institution's motto, "to immortality."

As if to reaffirm its tradtional establishment outlook after electing Yourcenar, the academy also chose journalist and author Michel Droit. One of the most right-wing staffers of the conservative newspaper Le Figaro, his major claim to fame is that he is the only journalist that president Charles de Gaulle ever allowed to interview him on television. At age 57, Droit becomes one of the institution's youngest members.

The academy, founded by Cardinal Richelieu in 1635, was intended from the start to assert the dominance of men in French letters in a period when French culture was dominated by female-run aristocratic literary salons in which extremely precious language was the dominant fashion.

The late novelist Colette, France's most distinguished modern woman writer, refused to run for a seat, saying there was no question of her applying to a club in which women had to pay house calls on men to be admitted.

The first woman allowed to contest won a single vote in 1970. In 1975, two women killed each other's chances by splitting the vote in a bid for academic immortality.

Yourcenar appears to have turned disadvantages to advantage in the highly ritualized politics of academy electioneering. Her 40-year absence from the Paris literary scene meant that she had a minimum of personal enemies.

By not campaigning for herself, she forced Giscard to throw his government's considerable weight behind her if he wanted to be able to acclaim the breakthrough for women. Academy member Alain Peyrefitte, who is also Giscard's justice minister, in effect served as her campaign manager.

The objection that Yourcenar had lived for so long in North America was neatly turned by recalling the precedent of the award last year of the Prix Goncourt, France's prestigious literary prize, to an Acadian woman writer Antonine Maillet for her account of the struggle of the original settlers of New Brunswick to stay French in North America. Yourcenar's advocates drew an analogy with Yourcenar's own successful struggle to maintain the purity of her French in an American environment.

Even so, there was so much vocal resistance that the comfortable margin of Yourcenar's victory came as a surprise. After admitting that many of her translations are better poetry than the classical Greek originals, academician Jean Guitton reproached her for abandoning her Catholic background and concluded, "The greatest homage one can pay a woman is to admire her without electing her."

Yourcenar's translations from English include works by Virginia Woolf and Henry James and the collection of spirituals, "Deep River, Dark Stream." She has been politically active in her adopted America, often demonstrating and writing congressmen on environmental issues and against the Vietnam war.

Born in Brussels of a Belgian mother and a French father, she has been a member of the Belgian royal academy since 1970, along with several other women. Yourcenar's extensive travels were restricted by the illness of her constant companion, Grace Frick, who died a few months ago. Frick translated Yourcenar's works into English.

In a French television interview rebroadcast tonight, she was questioned about the sense in her writing of the tragedy of life and whether she regretted not having children. "To leave behind books," she said, "is even more beautiful than leaving behind children because there are far too many children."