"Phantasm" will not be remembered as a masterpiece of the horror genre, but it sustains a gauche, hokey, desperately improvisational charm. Written, directed, photographed and edited by 25-year-old Don Coscarelli, this cockeyed bogyman story is about as frightening as a round-robin of ghoulish tales overheard at a slumber party. It entertains through a half-facetious juvenile gusto.

"Phantasm" arrives today at Washington theaters already a minor box-office sensation, having grossed over $6 million on a production cost that probably didn't exceed half a million. It may also earn a special niche of affection alongside other irresistibly silly exploitation "classics" like "I Was a Teen-Age Werewolf," "Bucket of Blood" or "Little Shop of Horrors."

Coscarelli's scenario subjects two orphaned brothers, teen-age Mike (Michael Baldwin) and 20-ish Joady (Bill Thornbury), to a frenzied, redundant succession of perils encountered snooping around the grounds of a sinister mortuary and cemetery.

Mike and Joady are particularly threatened by a jut-jawed, hulking attendant at the mortuary, embodied amusingly by an actor who has taken the name "Angus Scrimm" for the occasion, but is better known as Rory Guy, an experienced character actor and free-lance writer whose byline may be familiar to readers of record liner notes.

Coscarelli doesn't so much "develop" the premise as ricochet between settings, keeping the shocks coming and reinforcing them with garish, violent details. The showiest lethal device is a flying silver orb which bores out the skulls of trespassers.

It sounds grislier than it plays, because Coscarelli frequently lacks the skill or resources to endow his effects with a persuasive illusion. Nevertheless, this very limitation produces some daffy highlights - liek the delightful passage where Mike and Joady are required to bulldog an imaginary giant insect into submission.

The scene is set when Mike breaks into the mortuary, gets discovered, beats a hasty retreat and slams a door on his pursuer's hand. The protruding fingers wiggle wickedly. Mike slices them off. They ooze a mustardy blood, but he scoops one up and takes it home anyway.

Kept in a wooden box, the severed finger metamorphoses into some form of oversized, crocheted bug which hops onto Mike's head. Joady snatches a towel, pulls the angry bug off, and the pair-stumbles to the kitchen, wrestling with a now ferocious towel. Joady succeeds in shoving the bug down the garbage disposal. But it's hard to kill. Finally, he must grab a kitchen knife and repeatedly stab the disposal in order to finish off the pest.

Nothing else in "Phantasm" surpasses this sidesplitting battle to the death with homely and funny-looking props. However, its spirit is always within reach. Confronted by obvious failures of illusion, Coscarelli boldly pretends that the illusion has been achieved. For example, he insists that Mike and Joady are being pursued by a driverless car although a driver is clearly discernible in most of the shots.

The situations are consistently gratuitous but occasionally hilarious.Although the beginning suggests that the scares might grow out of adolescent sexual apprehension, the scenario has no dramatic integrity in the last analysis.