When Philadelphia reptile dealer Henry Molt met his shipment of almost 1,000 iguanas, crocodiles, pythons, death adders and monitor lizards at a New York pier, he was quizzed at length by Customs agents about why such rare reptiles arrived by way of Switzerland.

Molt's answers did not satisfy Customs, and an agent was sent to Philadelphia to check Molt's records. The agent was not satisfied; there was something fishy about the way Molt kept records, about the documentation that the rare tortoises from Madagascar and the even rarer iguanas from the Fiji Islands were legitimate.

Pressed for more answers, Molt gathered up the iguanas and lizards he had not sold to game ranches, zoos or private collectors for up to $1,000 apiece and bolted for the Pine Barrens of New Jersey where he killed and buried the reptiles. Right behind him, Customs agents dug up the evidence and took their case against Molt to the Justice Department.

Two months ago, Molt was sentenced by a federal judge in Philadelphia to nine months in jail for criminal violations of laws protecting the growing number of endangered and threatened species of animals the world over. He is now in Allenwood Prison in Pennsylvania.

Molt is one of a growing number of dealers in rare animals, especially reptiles and birds, who are being fined and imprisoned for smuggling animals into the United States from tropical habitats and for illegally dealing in domestic animals protected by the Endangered Species Act of 1973.

In the last 18 months, federal courts have imposed 23 jail sentences of up to five years and another 15 of up to five months, levied fines of more than $125,000 and placed dealers on more than 130 years of probation for violations of wildlife statutes.

"The courts have begun to treat this illegal dealing in rare wildlife as a serious criminal activity," said Kenneth Berlin, chief of the two-year-old Wildlife Section of the Justice Department's Lands Division. "We are now getting stiffer sentences, substantial fines and longer periods of probation for wildlife violations."

Berlin estimates the illegal traffic in rare wildlife in the United States at between $50 million and $100 million a year. He said that between 25,000 and 50,000 parrots alone are smuggled into the country every year from Mexico and Central America, that another 100,000 poisonous snakes are illegally shipped through the mails every year and that the traffic in such endangered and threatened domestic birds as bald eagles, scissortail flycatchers, broad-winged hawks, Carolina parakeets and California condors is on the increase.

The largest illegal traffic is in reptiles and birds, in part because of the proliferation of game ranches and private collectors and in part because many species are nearing extinction.

"Some people just want the rarest things and don't care how they get them," Berlin explained. "They don't necessarily make good pets but people want them and will buy them."

The largest traffic in illegal wildlife is in parrots and macaws, whose numerous, exotic and colorful species are being plundered for private collectors in record numbers. Rare parrots and macaws fetch as much as $10,000 apiece and even more abundant species like the yellow-headed Amazons go for $1,000 each.

Not long ago, a dealer was arrested smuggling 250 parrots he said were bred in captivity fron Mexico to California. An investigation found the parrots were so rare that only a few had ever been bred in captivity. "But he wanted us to believe that he had 250 of this handful in the back of his truck," Berlin said.

Another dealer in San Diego was caught with 32 yellow-cheeked Amazons he said he bought from a breeder in Florida. To prove it, he produced an invoice which was turned over to the FBI for handwriting analysis. The invoice turned out to be written by the dealer.

Since he was already on probation for smuggling 18 parrots into the country a year before, he was ordered to jail on the spot. "The judge didn't even let him go home to get his toothbrush," Berlin said.

The illegal wildlife trade is being fueled by the increasing number of private collectors. Berlin said that in metropolitan Philadelphia alone there are 500 serious collectors of rare reptiles. He said the membership of the American Federation of Aviculturists (breeders of birds) is now more than 50,000 nationwide.

The many collectors and hobbyists have bid up the price of rare wildlife so high that illegal dealing and smuggling have become worth the risk to some dealers. One Justice undercover operation, a story goes, turned up a man who hired the entire army of a small tropical country to go into the jungle and capture rare birds for his private collection. But officials said they were never able to prove it.