Just when you thought you'd read the final installment of "A Hitchhiker's Guide to the Galaxy . . ."

Or seen the final TV repeat episode . . .

Or heard the last radio show . . .

Or spun the final record album . . .

There is more. True to amoebic form, the Hitchhiker story will reproduce itself as a movie, is available as a home computer software game and, this month, the final book sequel, "So Long, And Thanks for All the Fish," will be in the bookstores.

Is there no limit?

"Well, we don't have a braille version and we don't have Hitchhiker's Guide dental floss," says Douglas Adams, the source of this meandering science fiction mock epic. In town to read excerpts from his works to college audiences, he is trying to conform his 6-foot-5 body behind a small Ramada Hotel breakfast table, lost somewhere in the outer space of Rockville.

He is just back from the ionosphere known as Los Angeles, where he has been negotiating the film project with Ivan ("Ghostbusters") Reitman, who has bought the option on the rights.

"Progress on the film is perceptible to the human eye. The biggest hurdles are finding a script we can all agree on and a director who can cope with the technical effects yet go easy on relentless, wonderful special effects . . . also if he could be English that would be good as well," says Adams, who is and looks English -- imagine Sid Caesar playing an overaged, wayward but mannerly British schoolboy, dressed in jeans and leather jacket.

The film, which begins production early next year, is the crowning glory of the "Hitchhiker" phenomenon, which has taken the form of radio serialization, a record album, a book with three sequels ("So Long" being the third sequel), stage adaptations (including a version that had the audience sitting in a circulating hovercraft) and a BBC television series. For the insatiable cult followers, there is the aforementioned software program -- in which you can participate as Arthur Dent, one of the story's characters.

"Curiously, the American audience sees the 'Hitchhiker's Guide' as being very English," says Adams, who seems to enjoy observing the varied reactions to "Hitchhiker" with a kind of omniscient benevolence. "And the English audience actually sees it, to quite a large extent, as being quite American. So it exists somewhere out in mid-Atlantic."

He admits to having done "pretty well" by all this. He lives in a 10-room flat in London, is dating a London barrister (too busy to get married just yet) and owns several computers. The 32-year-old son of a theology student and nurse has done well enough to "sit around the rest of my life if I was frugal and shrewd and invested it, but not enough to live off at the same time."

Certainly well enough to hire a secretary. He winces about the secretary, with the inbred British fear of seeming too big for one's boots. The secretary is not there because he wants to appear a big shot, he emphasizes, but merely to allow him to write undisturbed and "shield me from all the people who can't do their jobs properly."

"Writing is . . . pain. There's a process of months and months of just thinking 'I can't do it' and running away and then being virtually forced at gunpoint to do it in a hell of a hurry right at the end. Which is stupid. I wish I could educate my way out of that."

As far as the milking of one story goes, "Hitchhiker" is a good narrative cow. Arthur Dent, an innocent earthling minding his own business and still in his pajamas from that morning, is told by a visiting extraterrestrial, named Ford Prefect, that an unpleasant race called the Vogons intends to demolish the Earth in a few minutes to make way for a hyperspatial expressway. It's one of those Thursdays.

The Earth, as Dent later discovers, is (or was before it blew up) in fact an oversized computer anyway, built by pandimensional, extraterrestrial beings (in the form of mice) to ask the ultimate question that will help them understand the true meaning of life, the universe and everything. They had programmed a computer earlier to give them the answer to life, the universe and everything, but were given the rather unsatisfactory answer of 42. The problem, said the first computer, is you don't really ask the right question. Hence, computer Earth. The plot doesn't so much thicken as get more parenthetically complex.

"Hitchhiker" has what you might call cultural follow-through. Unlike the "Star Wars" anti-intellectual, gee-whiz attitude to technology, "Hitchhiker" finds the space age the best backdrop with which to reveal the universe of foibles that a human being represents. "Hitchhiker," with its sardonic twists and turns, makes you all too aware of real life. "If Jonathan Swift were alive today, he might well have written 'Hitchhiker,' " says Adams.

Take the Vogons, for example. They may seem to be from another galaxy, but they're nothing we haven't met in visits to the average government bureau. Vogons, writes Adams, are "not actually evil, but bad-tempered, bureaucratic, officious and callous. They wouldn't even lift a finger to save their own grandmothers from the Ravenous Bugblatter Beast of Traal without orders signed in triplicate, sent in, sent back, queried, lost, found, subjected to public inquiry, lost again, and finally buried in soft peat for three months and recycled as firelighters . . . Anatomical analysis of the Vogon reveals that its brain was originally a badly deformed, misplaced and dyspeptic liver."

Adams seems to have touched more than just a response: for many of his devotees, the series represents a permanent perspective on existence:

* Hitchhiker aficionados frequently show up at Adams' book signings carrying towels (a towel, according to "Hitchhiker," is "about the most massively useful thing an interstellar hitchhiker can have . . . any man who can hitch the length and breadth of the Galaxy, rough it, slum it, struggle against terrible odds, win through and still know where his towel is, is clearly a man to be reckoned with").

* Someone may well show up at your next Halloween party dressed in pajamas and a bathrobe. It is your cue to grin knowingly and say something like, "Come in, Arthur Dent."

* In Washington, Daniel McGunagle, a 35-year-old government employe (non-Vogon) with the Food and Drug Administration here, admits to making cryptic references to "Hitchhiker" on the telephone to innocent callers. He also hosts "Hitchhiker's Guide" parties, where you can either watch the whole television series or listen to the whole radio series. Dress is optional.

* "People come up to talk to me who actually know the text better than I do," says Adams. "And they just talk in quotations [from the book] , which can get kind of annoying . . . There was a phase when people were phoning me up all the time . . . one guy actually claiming to be Zaphod Beeblebrox [a character in the story ]. That got very wearying."

The anecdote of how Adams conceived the "Hitchhiker" idea threatens to become cult legend. As a young Adams (circa 1971) lay drunkenly under a large starry-nighted sky in an Innsbruck field in Austria, a copy of the "Hitch Hiker's Guide to Europe" under him, he decided if someone were to produce a similar guidebook for the rest of the universe, he'd be the first to go. He promptly forgot the idea and went to Cambridge University to take a degree in English literature.

After graduating in 1974, he played starving writer, doing "bits and pieces" for TV and radio and landing such odd jobs as bodyguard for Qatar's royal al-Thani family in London ("You had to sit outside the hotel room for 12 hours a night . . . guarding this gaggle of princesses, all immensely fat. They'd spend all their days in fabulous hotel suites ordering hamburgers and watching television").

He wrote six different drafts of what was originally called "The Ends of the Earth," all of which had the world blowing up, each time for different reasons. The Innsbruck idea returned to him and helped him find the perspective: why not create an extraterrestrial researcher for a book called "The Hitchhiker's Guide to the Galaxy"?

The BBC gave him the green light for the project gingerly and aired the drama in March 1978. In a "huge blaze of no publicity at all," as Adams puts it, it was successful. However, when Adams wrote a book version the following year which hit the number one spot of the London Sunday Times best-seller list in its first week and stayed there, "we realized it was a lot bigger than anybody had thought."

The BBC commissioned Adams to work on a television version, which was produced in 1981, finding its way to American public television the following year. The books made best-seller lists across the country, including the New York Times'. In the meantime he managed to find time to be script editor for the popular British "Dr. Who" TV series.

Contrary to pandemic rumor, he is not a veteran of the British Monty Python comedy troupe (he collaborated briefly with Pythoner Graham Chapman once, in a comedy project that was later aborted).

"I've tried to explain exactly what that's all about to journalists but I still read their article a couple of days later that I've written X number of episodes for them or something . . . Mention Monty Python and you just ring a bell. Maybe I shouldn't mention it at all."

Modern times seem to have caught up with his method of writing. "So Long and Thanks for All the Fish" was written virtually screenplay-style: in dialogue only, before Adams added the descriptive prose ("I think there's a certain good discipline getting as much of the story told in dialogue as possible. I think it makes for better storytelling"). He speaks of his next "screenplay," rather than "novel." He mentions "collaborations" he wants to try, rather than subject matter he'd like to tackle, and he admits to being addicted to his computers.

"The technology is not in itself bad, just the use made of it. When invention becomes the mother of necessity, that's when I draw the line."

Although Adams wants to avoid science fiction projects, although he'd rather trace his literary lineage to Dickens than to Kurt Vonnegut, although he seems to have explored his extraterrestrial themes in every conceivable way, it's hard to stay away from the genre.

"The curious thing is, as soon as I start to think of a story and other elements to introduce to sharpen it, somehow science-fictional things begin to creep in at the edges."

"Hitchhiker" fans will no doubt be glad to hear that.