THE PROPRIETOR of the Tropical Restaurant in Peru's Amazon jungle is a big, blond, crookedly built man with fingers like bratwursts and worn tattoos on his wide forearms and swollen ankles. He swears profusely and very profanely that his ceviche is the best around.

Considering that Iquitos, a city of 150,000 people and home of the Tropical, is about 400 miles from any other place of size, Paul Hittscher's boast probably is justified. His ceviche, nevertheless, in the opinion of the oil roustabouts, drug smugglers, missionaries and jungle camp directors--his colorful clientele--is the jewel of the menu.

This preference surprises Hittscher, who says he opened the Tropical because "we needed a damn good steak and a joint where you could get real American powdered mashed potatoes." He adds that he is not disappointed because "the fish in the Amazon are real good, some real monsters."

Firm, large-boned white fish make the best ceviche, and in the paiche, a six-foot-long monster, and the dorado, the Amazon sports two of the best ceviche fish in the world. The dish is Peru's answer to sushi; it is a raw fish offering in marinade spiced to one's pleasure. In the Amazon, the dish's coolness in temperature and tingling sensation make it as necessary to one's diet as brandy in the Alps.

Restaurants in Lima, Peru's capital, make their ceviche primarily of sea bass, and specialized ceviche bars will use almost anything that swims or soaks, including clams and mus- sels. But at the Tropical, it's only paiche or dorado, "no fancy stuff," Hittscher explains.

To prove his claim, Hittscher claps his fleshy hands from his corner table and tells the waiter to bring a ceviche "but first another whiskey and soda." Hittscher points to his enlarged ankles and laments "too much whiskey and too much sitting."

"This is the jungle," Hittscher wants his guests to know, a way of explaining that the wait for food may be a long one. Around his feet lizards roam the linoleum floor feasting on insects that frequent the roadside restaurant along with a rogues' gallery of customers.

"See some guy like that?" Hittscher nods his head slightly toward the table at the rear where two young men are seated, fondling American currency. "Don't stare. Someone like that, they'll kill you and drop you in the river and no one cares."

The Tropical is a haven in Iquitos, a miserable city where rutted streets gradually become impassable as garbage mounts and sickeningly ripens in the tropical sun. An acrid plume of smoke--jungle and wastes burning--hugs the city and is shunted aside only momentarily during the afternoon rains.

The city once was the capital of Peru's rubber boom, then in the 1970s it was a base for oil exploration. Both efforts left nothing more than broken dreams. Now the city serves as a major transshipment center on the route of cocoa leaves from the highlands of Peru to the laboratories of Colombia.

Hittscher says he came to Iquitos a decade ago when his sea ran out. A ship captain for the very rich, the German seaman sailed a yacht 2,300 miles up the Amazon to sell it to an expatriate American who wanted to use it as a tour boat. Hittscher stayed on as the helmsman until the ship burned and sank. Then he became a restaurateur.

He is slightly embarrassed by the Amazonian inefficiency of his staff, and he barks, "Rapido. Whiskey, soda, ceviche. Rapido." The locals query him on where he's going in such a hurry.

As he good-naturedly suggests destinations for them, the ceviche finally arrives in the hands of Hittscher's wife, Edelmira. "I call her Edy," Hittscher instructs, "but you call her Senora Hittscher." He laughs with a phlegmy roar that rumbles his huge belly.

A petite, vibrant woman, she ignores her husband and gently sets down the plates without spilling the marinade. The white fish is cut into bite-size chunks, dressed with small slices of onions, freckled with herbs and afloat in a pond of juice. Its smell is like perfume of fresh-cut grass, dominated by the bits of cilantro. As pillars on the plate are half an ear of corn and half a sweet potato.

Paul Hittscher turns serious as the tasting is about to begin. "You can't get it like that in the States," he points out. Obviously, paiches don't run in the Potomac, but, Hittscher explains, the key to making ceviche in the United States is hitting upon the right combination of lemon and lime juice. He suggests using a marinade with equal portions of the citrus fruits.

Edy Hittscher scolds her husband and tells him to let his guests eat. He does.

"Do you like it?" he demands to know, his weathered face perched over the table waiting for an answer. He suggests that unhappy customers are invited to swim in the mighty brown river with a less meaty fish--the piranha.

His threat is unnecessary; the ceviche is splendid, tangy and addictive like potato chips--one bite leads to another, then another. Where heat quells appetites, ceviche is more than an appetizer; it's a meal. The corn and sweet potato are meant to mitigate the spice, but they are ample side dishes.

"It's about time you're through," Hittscher barks as he grabs the marinade remaining on one plate. He pours it into a glass, swirls it around and swigs it in one motion.

"Leche de tigre. Tiger's milk. It's the best thing for a hangover." He winks, and he orders vodka to go with the remaining tiger's milk.

He toasts his guests, pleased that they love his ceviche, and he becomes nearly sentimental. He describes Iquitos with a profanity, then adds, "But my good friends, my good wife, and my good food make me happy." TROPICAL RESTAURANT CEVICHE (8 servings) 2 pounds white fish (halibut, sea bass, gray pike, sturgeon, yellowtail) About 6 lemons About 6 limes 3/4 large onion, thinly sliced, or to taste 1 tablespoon Chinese hot peppers, or to taste 1 teaspoon fresh cilantro (coriander), finely minced, or to taste Pinch garlic salt, salt and pepper 5 stalks celery, chopped, or to taste Lettuce for serving 4 sweet potatoes, cooked 4 ears corn, cooked

Cut the fish into bite-size chunks (make sure all the bones are out). Marinate for 1 1/2 hours in half-lemon and half-lime juice, making sure all the fish is immersed. In the last half hour add thinly sliced onions, finely cut Chinese hot peppers, finely minced cilantro, garlic salt, salt and pepper--all to taste. Add a generous amount of chopped celery to give the dish a taste of freshness. Serve on a bed of lettuce and place a half ear of boiled corn and half a sweet potato on each plate.