Arthur Koestler, 77, a writer on politics, science and ethics and the author of "Darkness at Noon," one of the most eloquent and widely read indictments of communism of the age, was found dead yesterday with his wife, Cynthia, in their London residence. Police said they committed suicide.

Mr. Koestler, who had Parkinson's disease and leukemia, was a supporter of euthanasia. In 1981, he was made a vice president of the Voluntary Euthanasia Society, EXIT, and wrote an introduction to its publication, "Guide to Self-Deliverance," a primer on how to commit suicide. Unlike animals, he wrote, man does not die "peacefully and without fuss in old age."

London police said the Koestlers died of a drug overdose and that a note had been found with them. Authorities declined to disclose what it said or other details. Mrs. Koestler, who was in her 50s, was believed to have been in good health.

Pat Kavanaugh, a director of A.D. Peters and Co., Mr. Koestler's literary agency, said the writer "was not a man who liked to operate at anything but his fullest potential. He had been very ill and I should think he found it intolerable."

Two years ago, a leader of EXIT was sentenced to prison for giving advice on how to commit suicide. That is a crime in Britain, although committing suicide is not.

Mr. Koestler, a Hungarian-born Jew, embodied the dilemma of many liberals of the 1920s and 1930s: a choice between seemingly weak and vacillating democracies beset by reactionary fascism on the one hand and by progressive and apparently humane communism on the other. Like thousands of his contemporaries, he joined the communist party. Like thousands of others, he turned against it. This experience gave him the material for a remarkable literary legacy of the times.

Mr. Koestler became a major figure in the worlds of politics and letters in 1940 with the publication of "Darkness at Noon," a novel about the destruction of an Old Bolshevik, Rubashov, in the Stalinist purge trials of the 1930s. Rubashov is executed after having confessed to crimes that he knew he had not committed. Although he abandoned truth even at the price of his life, truth dies with him, a victim of the tortuous and perverted imperatives of "the Party."

The book thus illuminates one of the oddest of the phenomena associated with communism. This is the willingness of its adherents so completely to submerge themselves in its goals that they are willing to revile themselves and their accomplishments, their whole careers in fact, in the service of what are perceived as the higher interests of the movement.

The book appeared within a year of the treaty between the Soviet Union and Nazi Germany that led to the outbreak of World War II. That pact drove many from the embrace of communism. Coming virtually on its heels, Mr. Koestler's book, a political tract as much as a work of art, had an enormous influence on many whose faith still hung in the balance. It is regarded now as a benchmark of its kind. More than a million copies have been sold.

In 1950, Mr. Koestler contributed an essay to "The God That Failed," a collection of pieces by former communists edited by Richard Crossman, a Labor member of Parliament. It is an account of how he himself chose to join the party and why, after seven years, he left it.

"I became converted because I was ripe for it and lived in a disintegrating society thirsting for faith," he wrote. "To say that one had 'seen the light' is a poor description of the mental rapture which only the convert knows (regardless of what faith he has been converted to).

"The new light seems to pour from all directions across the skull; the whole universe falls into pattern like the stray pieces of a jigsaw puzzle assembled by magic at one stroke. There is now an answer to every question, doubts and conflicts are a matter of the tortured past--a past already remote, when one had lived in dismal ignorance in the tasteless, colorless world of those who don't know."

He broke with "the party"--among its followers it is known by no other name, Mr. Koestler tells us--after he was imprisoned for three months by the fascists in Spain during the civil war there. There was fear and pity, he said, and a special fear, "not of death, but of torture and humiliation and the more unpleasant forms of dying . . . ."

"The lesson taught by this type of experience, when put into words, always appears under the dowdy guise of perennial commonplaces," Mr. Koestler said. " . . . that man is a reality, mankind an abstraction; that men cannot be treated as units in operations of political arithmetic because they behave like the symbols for zero and the infinite, which dislocate all mathematical operations; that the end justifies the means only within very narrow limits; that ethics is not a function of social utility, and charity not a petty-bourgeois sentiment but the gravitational force which keeps civilization in its orbit."

In the mid-1950s, Mr. Koestler abandoned politics, saying that he had contributed all he could to the subject. He wrote several books on science in which he developed the thesis that scientific progress is the result of the same unconscious processes that produce art, and not the result of rationalism and empiricism. Still later, he wrote of mysticism, parapsychology and drugs.

Under the urging of Dr. Timothy Leary, the apostle of psychedelics, he outraged his admirers by suggesting that the dilemma of aggression and survival might be solved by development of a "peace pill."

Arthur Koestler was born in Budapest on Sept. 5, 1905. His parents were Henrik and Adela Koestler. His father was a prosperous representative of German and British textile firms in the Hungarian capital until the outbreak of World War I in 1914.

After the war, the family moved to Vienna. The young Koestler planned to be a scientist but abandoned his studies to support his parents, who had become impoverished. He became a journalist and was a correspondent in the Middle East, an experience that led him to support the radical form of Zionism associated with Vladimir Jabotinsky and Menachem Begin. By the end of the 1920s, he was an editor with the Ullstein publishing house in Berlin.

On Dec. 31, 1931, he joined the German Communist Party. He lost his job, traveled extensively in the Soviet Union, and then settled in Paris. He went to Spain during the civil war as a correspondent for papers in London and Budapest. After his release from a Spanish prison as a result of British intervention with the authorities, he returned to France. He was imprisoned briefly by Vichy authorities and then escaped to London, where he joined the British army.

Mr. Koestler had lived in London since the war. In all, he wrote more than 30 books. Although he was staunchly anticommunist, he remained a critic of free enterprise and western social democracy, regarding western systems as the lesser of many possible evils rather than a panacea for the ills of mankind.

He was a notable and effective opponent of capital punishment in Britain. He was a founder of the Congress for Cultural Freedom, which later was discovered to have been funded, without his knowledge, by the Central Intelligence Agency.

Mr. Koestler's marriages to the former Dorothy Ascher and Mamaine Paget ended in divorce. In 1965, he married his third wife, the former Cynthia Jefferies, who had been his secretary. He leaves no immediate survivors.