"Alligator," the most amusing variation yet on the "Jaws" formula, finds plenty of room for incidental humor and romantic byplay while sustaining a breezy suspense plot. It centers on the efforts of a hard-luck police detective, Robert Forster, and a comely herpetologist, Robin Riker, to capture a hulking, ravenous gator lurking in the sewers of a midwestern city.

The entertaining aspects of "Alligator" don't come as a complete surprise. The script, written by John Sayles during the same period of Hollywood genre apprenticeship that also produced "The Howling," confirms his refreshing touch with cliched pretexts. A number ofjokes may be gratuitous fun -- the first victim is a municipal sanitation worker called Ed Norton -- but a far greater number are deftly integrated into the plot.

For example, the prologue prepares the audience for an outrageous dramatic irony: The marauding alligator is probably the heroine's longlost pet, a miniature reptile she named Ramon, who was flushed down the toilet years earlier by her irritable dad and grew to gargantuan sixe on a rich diet of toxic drain water and discared lab animals sent his way courtesy of an unscrupulous chemical company, Slade Pharmaceuticals. Sayles follows the premise to outrageous conclusions. After the beast surfaces, escaping a police manhunt by crashing up through the sidewalk, it goes on a climactic rampage at the estate of dastardly Slade himself (Dean Jagger, looking alarmingly dessicated), who happens to be throwing a huge wedding reception for his daughter.

Sayles and director Lewis Teague, who seems a resourceful manipulator of all key elements, have followed the cagey "Jaws" example of exposing their monster gradually. In fact, it's no doubt a more effective menace when largely submerged. Ominously wet, sleepy eyelids retract on a slumbering hulk of some kind moments before the beast draws a bead on the tempting posterior of Sidney Lassick, cast as a wretched pet-store owner who supplies and dumps animals for Slade. An intrepid yellow journalist is confronted by colossal jaws an instant before being chomped but has the stuff to activate his camera, which keeps flashing away. Nothing is left of the reporter, but his camera is fished out of the filtration tanks and provides the first documentary confirmation of the monster's existence.

As a rule, the victims are types who can defend themselves (up to a point) or who ask for trouble. For example, the always sinister Henry Silva turns up as an arrogant big-game hunter who gets more game than he bargained for while tracking the alligator up a garbage-strewn slum alley. The one exception -- and the most conspicuous misjudgment in the movie -- is a little boy whose victimization seems unacceptably nasty and preposterous, in part because it rests on the assumption that no one notices the 30-foot alligator lolling all day in a back yard swimming pool.

Robert Forster projects a battle-weary appeal as the seedy but stalwart hero of "Alligator." Looking worn and slightly punchy actually becomes him in this role. The romantic leads are, indeed, attractively conceived misfits -- an honest guy with a loser's reputation and a brainy girl whose profession tends to keep men at a distance.

The casting is sometimes eerily playful. Silva and Jagger are joined in substantial supporting roles by Michael V. Gazzo, playing Forsterhs superior. The venerable gangster hulk Mike Mazurki appears receiving guests at the entrance of the Slade estate. "Could I see your invitations?" he politely insists when Forster and Riker drive up frantically, hoping to prevent the massacre already under way at the reception.

Silva brags about his exploits to a TV interviewer whose striking but faded visage I couldn't identify, although the names of several once-promising actresses wafted in and out of speculative focus. During the closing credits I was stunned to discover that it had been Sue Lyon. In a way, this revelation lingers as the most authentic shock in a agreeably contrived shocker.

"Alligator" is sufficiently witty and skillful to overcome most of the obstacles posed by skimpy monster-thriller budget -- $1.5 million according to Cinefantastique. Although the movie easily outclasses this month's shipment of horror melodramas, it may not be able to overcome the absence of effective advertising. There were only a handful of enthusiasts sharing the fun when I sampled "Alligator" at the Centre in Alexandria. Don't be surprised if you're obliged to catch this diverting castoff on the rebound.