The Gaboon viper that bit young Lewis Morton Monday night is one of the deadliest snakes in the world--a sluggish, thick-bodied serpent whose venom, injected through 1 1/2-inch fangs, kills an untreated victim within minutes.

Native to the jungles of western, central and eastern Africa, and most populous in the mountains of Cameroon, it grows to nearly six feet long and to the thicknness of a man's calf.

It is also as beautiful as it is deadly, patterned in spectacular hourglass geometry of yellow, blue and black reminiscent of a tubular Oriental carpet on the move.

The Gaboon viper, however, is not often on the move.

"They're really not very bright," said Dr. Dale Marcellini, the National Zoo's curator of herpetology. "They just lie there."

Once aroused, however, they strike with tremendous force, lifting the entire forebody and flat, wedge-shaped head and injecting venom that attacks both the circulatory and nervous systems. They may continue agitated hissing for hours.

Those Marcellini looks after, a male and female roughly 4 1/2 feet long, lay coiled and quiescent yesterday in separate portable basement cages, home and apparently unhurt after their theft Monday night, as the zoologist sought to defang reptilian myths he says defame a scaly nation of innocent creatures.

"Everybody gets excited about snakes, but if they would just leave them alone they wouldn't get in trouble," he said. "In 10 years here we haven't had a bite. I've never heard of a captive snake biting anyone unless some macho character was trying something stupid. This business was the stupidest of the lot. Can you imagine trying to handle one of these things with your bare hands?"

Messing with a Gaboon viper, Marcellini says, is about like kissing a cobra. Whoever tried it Monday night "must have thought, despite the signs which identify the vipers as venomous , that they were too quiet to be scary. And of course, they're a spectacularly beautiful animal."

Marcellini said the burglar or burglars who took the snakes, each valued at $300 to $350, apparently entered the reptile house by breaking a six-by-eight-foot window beside the front doors sometime Monday night ("probably with a rock or some kind of fence post"). The intruders also shattered the glass cage holding the zoo's two cottonmouth water moccasins.

The cottonmouth, Marcellini said, is "a very aggressive snake," unlike the Gaboon viper, and "probably reared up and started hissing. That may have scared them off."

The cottonmouth remained in his cage, as most animals raised in captivity will do if given the chance, Marcellini said.

Also broken during the night was a six-by-eight foot double glass window in a crocodile house adjacent to the main reptile house, Marcellini said, "but that may have been a separate incident." He estimated the total damage to zoo facilities at about $2,000.

Marcellini says the National Zoo has only eight types of venomous snakes--a collection kept purposely small to minimize "the risk of something just like this. The more dangerous snakes you have, the more chance you have of somebody getting bitten."

Zoo procedures, he said, require clear labeling of all cages containing venomous snakes, complete with a red card listing the type of snake and the antidote required to treat a bite.

The snakes are never handled with less than two persons present. Should either be bitten, he is never more than a few yards from an alarm button that will summon help. He is then rushed to George Washington University Hospital together with the red card and a vial of the appropriate antivenin from the reptile house refrigerator.

"We've timed it over and over and even during rush hour we're never more than 10 minutes away from the emergency room," Marcellini said.

The zoo keeps only about 15 10-cc vials of antivenin on hand, each priced at between $100 and $150, Marcellini says, but has arrangements for obtaining more from other zoos in a hurry.

Monday night, he says, he had already cabled inquiries for more to South Africa, where it is manufactured by injecting small quantities of viper venom into horses and extracting their subsequent resistant blood.

The zoo's snake safeguards, however, are designed for its employes.

"We don't assume anybody's going to break in here," Marcellini said. "We've never had a break-in in the reptile house in 50 years."