KAHUZI-BIEGA NATIONAL PARK, ZAIRE -- There are three countries in the world where one can get close to wild mountain gorillas.

American actress Sigourney Weaver is making a movie with the ones in Rwanda this summer. In Uganda, torn by civil war, humans have used machine guns to kill gorillas.

One can also find gorillas at this 1.5 million-acre park in Zaire. But finding an amiable gorilla gorilla graueri is an entirely different affair.

It took seven hours of walking, crawling, climbing, wading and picking stickers out of our elbows before a gorilla deigned to make his presence known. A large "silver-back" male was somewhere above us, hidden behind bamboo and green ferns, when it screamed.

It was a guttural howl, intimidating, heavy on bass -- the kind of outburst one might expect from a very large linebacker having shut a car door on his thumb.

Our guide, Mankoto Ma Oyisenzoo, a Zairian park ranger and gorilla expert who had "habituated" this particular screaming primate to the presence of human gawkers, didn't like the gorilla's aggressive tone.

Nor did the ranger approve of our location vis-a-vis the 400-pound male and his family, directly above us on a steep hillside. By the sound of it, the silver-back was breaking bamboo trees like toothpicks.

"If he charges, he may slide and fall on top of us," Oyisenzoo explained in a calm whisper, referring to slippery ferns on the hillside.

Our 11-member party of French, Turkish, Canadian and American tourists silently agreed that it was not be the wild gorilla experience we wanted. We retreated to try a different route.

There are 351 mountain gorillas in the wild, according to the African Wildlife Foundation. Two-thirds of them roam the Virunga mountains, a thickly forested chain of six extinct volcanoes that run across Rwanda, Uganda and Zaire. The remaining gorillas are in western Uganda's Ruwenzori range.

According to Oyisenzoo, most of the world's largest primates are here in Zaire. Neighboring Rwanda, however, has attracted fame for its gorillas. The reputation derives from American naturalist Dian Fossey, whose research there showed mountain gorillas to be shy, often gentle and intelligent vegetarians. While writing "Gorillas in the Mist" and making National Geographic films, she became a world-famous protector of the endangered animals.

It was in Rwanda, on Dec. 27, 1985, that Fossey was murdered with a machete. Who did it remains a mystery. And it is in Rwanda that Warner Brothers and Universal Pictures have this summer rented the Parc des Volcans -- and all the gorillas therein -- for a film about Fossey starring Sigourney Weaver. The park has been closed to tourists for two months for the filming.

"Gorilla tourism" in Rwanda is an important business. It is the primary reason for a tenfold increase in that country's tourism in the past decade. It takes weeks or even months to get an appointment to see a gorilla.

Not so in Zaire. If you show up early at this park and have 1,500 zaires (about $13) in your pocket, ranger Oyisenzoo will take you for a long walk. As we flanked the family of gorillas, the silver-back continued to howl and break bamboo. Oyisenzoo said something unusual seemed to be bothering him.

A specialist in the feeding habits and movement patterns of gorillas, it was Oyisenzoo, himself, who had habituated this particular family to the sight and smell of human beings. It took about nine months, he said. At first, he went alone into the forest and, keeping a distance of several hundred feet, allowed the nervous family to grow accustomed to his presence. Each day he went a little closer. Finally, he could move within 15 feet of the silver-back.

Then Oyisenzoo started bringing in tourists. Normally, Oyisenzoo said, the family will tolerate a group of up to 12 people, as long as they keep quiet and stay close together.

When our group had walked a semicircle around the gorillas and was finally above them, Oyisenzoo ordered his two machete-wielding assistants to cut a path so we might take a look. The ranger said the animals probably would relax because they knew they could always escape downhill.

The silver-back, however, did not relax as the assistants began to hack bamboo with their long knives. Sounds of tree-bashing and high-decibel grunting echoed through the forest. He did not sound as though he wanted guests.

The ranger's assistants, experienced forest guides from a local Pygmy tribe, began to argue with Oyisenzoo about the wisdom of getting any closer.

After a 15-minute huddle with the Pygmies, the ranger sheepishly turned to his muck-spattered followers and said that we would have to turn back. "The silver-back is too aggressive," probably from having been in a fight with another gorilla recently, Oyisenzoo speculated. It was unusual, he said, to walk so many hours and only hear gorillas, he added.

Most aspiring gorilla watchers in Zaire stay at the Hotel Residence in the town of Bukavu, which is near Kahuzi-Biega Park.

It was there at the outdoor bar, on the evening after our unsuccessful outing, that other tourists were celebrating. They had seen a family of 10 gorillas sitting in bright sunlight in a meadow. They had had to walk only two hours. The gorillas had been very nice.

"I have never seen anything so big," said Helen Wilson, a perky biology teacher from Fresno, Calif. She said she had taken many excellent close-up pictures. "It was the most wonderful experience of my life."

I bought her film.