"Saturday Night Fever" assaults you with a flagrantly foul-mouthed script and coarse viewpoint. This persistent coarseness echoes rather than illuminates the ignorance of the characters, a forlorn group of young people residing in Brooklyn whose social life centers around a neighborhood disco joint.

Screenwriter Norman Wexler and director John Badham seem as desperate to make the audience as do the kids at the club when attempting to make each other. They affect the same idiom, but it limits their expressive range just as severly as it limits the kids'.

Among other graphic titillatons, the movie is very keer on backseat sex, which is given two encores the last time with a gang-rape variation. And the movie as a whole resembles these episodes: It's also furtive and overheated, not to mention bungled and unsatisfying.

The screenplay was inspired by a piece of lifestyle reportage, Nik Cohn's new York magazine article "Tribal Rites of the New Saturday Night," an account of the mores and dating habits of working and lower-middle class kids in the Bay Ridge section of Brooklyn. The movie was shot for the most part in Bay Ridge, but it scuttles the article's sense of historiacl persepective and tolerant human interest.

"Saturday Night Fever" quickly degenerates into an urban exploitation film, reflecting much of the confusion and hypocristy of Paramount's earlier tawdry entry in the field, the movie version of "Looking for Mr. Goodbar."

The central character in "Fever" is a 19-year-old big fish in a small pond named Tony Manero, played by TV star John Travolta, getting an excessive buildup in his first feature starring role. Tony lives with his almost clownishly conventional Italian-American family (pious mom, gruff pop, nervous granny, plus the tendentious bonus of a favorite elder brother who creates a scandal by leaving the priesthood), clerks in a paint store and unwinds Saturday nights and with his cronies at the disco, where he is regarded as hot stuff, the most accomplished and desirabel male dancer among the regular customers.

The title suggest affinities with the early '60s British movie "Saturday Night and Sunday Morning," but the tough young working-class protagonist of the story, which helped make Albert Finney a star, would have chewed up Tony and spit him out in about six seconds flat.

"Fever" is a rabid variation on a theme that customarily evokes pathos - the loss of adolescent illusions and self-deceptions. The material has more in common with Fellini's "The Last Picture Show" and Robert Altman's recent "3 Women." It's regretable that "Fever" lacks all traces of their ironic humor and tenderness.

After an hour-and-a-half of grungy obviousness, Badham and Wexler seem to breeze right past the moment of dramatic truth that would redeem the material - Tony's realization that he has been nothing but a big fish in a small pond, which occurs when he and his partner win dance contest that he knows is rigged for neighborhood favoritism, the superiority of a competing Puerto Rican couple being painfully apparent to him.

The movie would probably be better off if it ended as quickly as possible - even abruptly - in the wake of this revelation, which endows Tony with a trace of dignity and self-respect. Instead, the story staggers around for another quarter hour or so, inventing a tragic death that is apparently supposed to impact a lesson the hero has already learned and then patching things up between hero and heroine in a simpering fadeout.

The filmmakers may have been reluctant to face Tony's moment of truth squarely because of the nature of their simultaneous buildup of Travolta. Tony's revelation happens to confirm what is apparent from Travolta'f first turn on the disco ballroom floor; at no time are we watching a young man who demonstrates a natural or exciting flair for dancing. The very idea that Tony would be considered the tops is a devastating comment on the limited experience and horizons of his peer group.

Nevertheless, the movies is concerned with promoting Travolta as a sex symbol at the same level on which Tony is adored by the dumpy and/or slutty girls who appear to frequent the Bay Ridge disco. As a result, one heard a lot of drivel about how hard Travolta worked to perfect his uninspired strutting and preening and hand-jiving. At the same time, the filmmakers are trying to cover for him by doing technical struts of their own, lighting the dance floor in a gauzy, filtered style taht obscures the movement and eliminating any sustained dance performance by cutting and framing figures disjointedly.

You never get on unobtrusive view of dancing in "Saturday Night Fever." Even the Puerto Rican hotshots are cut to teeth-flashing shreds after their few moments.

For what it's worth, Travolta is a more graceful figure on the dance floor than Karen Gorney, who plays the aspiring young woman he's attracted to. The film might have been more fun as a romantic comedy about the nature of their attraction, which resembles the relationship between Shelly Duvall and Sissy Spacek in "3 Women," with Travolta feeling intimidated by the largely fraudulent sophistication affected by Gorney.

The filmmakers certainly attempt to derive humor from the characters' ignorance, but it usually takes an unforgivably mean, crass form. With credits like "Joe," "Serpico" and "Mandingo" behind him, Norman Wexler is obviously not the writer you engage for the light comic touch. John Badham's direction comes as an unpleasant surprise; one wouldn't have expected something this brutal and garish from the director of "Bingo Long" and the TV film "The Law."

It remains to be seen what the picture may do to or for John Travolta. I have grace doubts about the romantic potential of a young actor whose most appealing expression is a kind of dumb vulnerability - Brian De Palma seemed to have the right idea when he cast Travolta as a stooge in "Carrie" - but for all I know it may generate an appeal as vast and inexplicable as the appeal of disco music.