Because Thomas Pynchon is so obsessively phobic about publicity, he caused the MacArthur Foundation a slight problem when it wanted to present him with a $310,000 ''genius'' grant. The novelist`s evasive tactics made it difficult for the foundation to contact him.

Pynchon is an even more forbidding and successful recluse than J.D. Salinger, who occasionally has been photographed outside his Cornish, N.H., bunker. By contrast, Pynchon is a migratory creature. He rarely has been sighted, much less interviewed. The last known photograph of the author was taken in the `50s for a college yearbook.

Pynchon`s address and phone number are known only to a precious few friends and associates, all of whom are apparently sworn to honor his fetish for privacy. Among them is his agent, Melanie Jackson, who was willing nonetheless to put the MacArthur Foundation in touch with Pynchon. Considering the size of the unrestricted ''fellowship,'' given to individuals to further their work, he would surely have been in the market for a new agent if she hadn`t.

Even so, there was speculation at the foundation about whether Pynchon would accept, because his disdain for literary prizes is almost as legendary as his aversion to publicity. But Pynchon not only returned the MacArthur Foundation`s call, he accepted it without hesitation, according to Ken Hope, director of the fellowship program. ''He was very congenial and gracious,''

Hope said.

Hope still doesn`t know the author`s address or phone number, though.

''I didn`t ask,'' he said, ''and I figured he`d tell me if he wanted me to know.'' His failure to ask may have led to the confusion last week, as foundation officials had listed Pynchon`s home as Boston. (Hope said there would be no difficulty getting the money to Pynchon; the installments will be sent to his agent.)

For more up-to-the-minute information about Pynchon`s whereabouts, they might have consulted Catherine Ingraham, a Chicagoan who wrote her doctoral thesis on Pynchon`s fiction and subscribes to ''Pynchon Notes,'' a little compendium of scholarly articles and gossip issued irregularly from Wesleyan University in Connecticut. ''He`s living somewhere in Southern California and sometimes in Mexico,'' Ingraham said. ''Everybody knows that.''

But recent rumors, she added, have him ''walking the Mason-Dixon Line, presumably doing research for his next novel.'' If that`s true, she said, it would suggest that he`s pursuing what she believes is a major theme of his work: ''a fascination with fictitious boundaries and edges that have profound political consequences.''

Despite his heroic, serpentine efforts to keep the lowest possible profile, Pynchon hasn`t managed to turn himself into a total cipher. This much is part of the public record: He was born on May 8, 1937, in Glen Cove, N.Y. He graduated from Oyster Bay (Long Island) High School and Cornell University, class of `58, where he was a student of Vladimir Nabokov.

Though he studied English, Pynchon had an insatiable appetite for science, especially elementary particle theory. After college, he worked as a writer for Boeing Aircraft in Seattle, and that experience, along with his consummate knowledge of physics, apparently provided him with a foundation for his three novels: ''V,'' published in 1963; ''The Crying of Lot 49'' (1966);

and ''Gravity`s Rainbow'' (1973), winner of a National Book Award.

Rejected for a Pulitzer Prize, reportedly because of incomprehensibility and obscenity, ''Gravity`s Rainbow'' is considered the finest, as well as the most labyrinthine, of Pynchon`s novels, the story of a disoriented American Army officer who gets caught up in a postwar holocaust of V-2 rocketry. Critic Frank D. McConnell called it ''one of the few truly great novels of the century, and at the same time one of the most disappointing, disturbing, maddening. . . . a melange of scientific jargon, Gothic lyricism, and comic-book pop dialect.''

Four years ago Pynchon published a collection of five early stories,

''Slow Learner,'' with an introduction that included some tantalizing autobiographical notes for cultists. From these and other sources, Helen Dudar described Pynchon, in an article for The Tribune, as a baseball, movie and pop-culture junkie. Despite reports that he is ''excruciatingly shy,'' she wrote, ''he is good company and a fairly accomplished cook, tending toward vegetarianism. He cuts his own hair, skillfully.''

Though many readers find Pynchon`s novels impenetrable, most critics and scholars praised his selection by the MacArthur Foundation. Frederick Karl, a professor of English at New York University who wrote extensively about Pynchon`s work in his book ''American Fictions: 1940-1980,'' called it a

''fitting and rare kind of award in this country.''

''Pynchon has created a distinctive language and voice for the American novel,'' Karl said, ''while avoiding all the usual means by which an American novelist achieves fame.''

Pynchon`s refusal to accept the 1973 National Book Award led to one of the more outrageous stunts at that usually august ceremony. With the author`s collusion, his publisher, Thomas Guinzburg of Viking, hired a comedian, Professor Irwin Corey, to impersonate Pynchon. ''He was wearing a dashiki and a gold medal,'' Guinzburg recalled. ''He launched into this splenetic doubletalk, stringing buzz words together very fast. . . . Since nobody knew what he looked like, it took the audience a couple of minutes to catch on that he wasn`t the real Pynchon.''