THE HOTEL TANIT in Hammamet stands on a long scallop of sand not far from the old fortress, its white bungalows shaded by palms and privacy shielded by thick groves of eucalyptus. From the terrace on the ocean side, the Mediterrean usually is a deep blue. But if you walk across the white beach and out into the water it becomes greener and greener until you are no longer walking but swimming and there is only that watery green light over the white sand.

The sea is perfectly clear to a depth of 15 or 20 feet, and when the wind is flat you can see the shadows of large schools of fish passing over the sands as they feed in the shallows offshore. In the early morning, fishing boats filled with men in billowing turbans set out from the beaches, sun-bleached blue and white boats with battered wooden hulls and gunwhales worn smooth from the nets. As themen troll through the schools, the fish in their panic churn the water to a broiling silver.

Later in the day the catch is sold in the narrow streets of the port, beneath the walls of the fortress now red in the slanting afternoon sun. After dark the shadows of the boats drawn once again on shore conceal in turn the warmer, darker shadows of lovers murmuring softly in tryst.

Hammamet marks the northern-most point of the Tunisian sahel--or seacoast, in Arabic--a seemingly endless curve of fine sand beaches backed by luxuriant gardens and posh hotels stretching far south past Sousse and Monastir. Without doubt, Tunisia's beaches are among the finest in the world. The climate is mild in the winter, not too hot in summer. Oranges and lemons ripen year round in the sahel's perpetual greenness.

In contrast to most Caribbean and European vacation spots, Tunisia offers much more than a sun-drenched escape from the northern winter. Wedged between Libya on the east and Algeria on the west, its foot in the Sahara and head on the Mediterranean, Tunisia is in many ways the heart of North Africa, a place where European, Near Eastern and Arab civilizations have clashed and fused. A sense of historical density pervades the sahel; it teems with architectural and cultural evidence of the centuries it has endured.

So many invaders. So many kings and emperors, caliphs and sultans. When Caesar created Africa Nova in 45 B.C., Carthage ruled the land. Then in waves came the Romans, Vandals, Arabs and Ottoman Turks, the Spanish, British and French. They left behind a heritage unmatched in North Africa: the coliseum in Dougga, superior to that in Rome; the graceful empire architecture and tree-lined boulevards of Tunis, transplanted by the French from Paris, Nice and Marseilles; the honeycombed medina of Sousse, built by the Arabs, inspired by Byzantium.

Today, Tunisia is more of an extended carnival than ever. Europeans descend in droves to vacation and do business. Arabs from the Persian Gulf come to live and invest their billions. Everywhere there is neon, everywhere TV antennas, nightclubs, skyscrapers and traffic jams. Streets have Arab names, French names, English names. Marriages with foreigners are forbidden, but women have equal rights under the constitution. Abortions are available on demand.

For all its modernity, Tunisia remains fundamentally Arab. You see it all around, in the calls to prayer, the mixture of modern and traditional garb, the noisy density of the streets, the souks where you shop. So to spend time in Tunisia is to do more than acquire a tan and recipes for rum drinks. It is to be immersed in an old and ambiguous culture undergoing rapid change, where western ways of thinking apply only sporadically and contradiction rules.

Trips to Tunisia all begin and end in the capital. Your plane arrives there, your plane departs there. And if you are heading for the coast, you would do well to spend a few days in Tunis, one of the most civilized cities in North Africa.

After the squalor and aggressive poverty of Marrakesh, Fez or Algiers, Tunis is uniquely gentle--gentle and, by Arab standards at least, even passive. The city seems almost too clean, its services too well organized, to be genuinely Arab. To walk the oak-screened promenades of the Avenue Habib Bourgiba, with its crowds of suited businessmen, flower stalls and cafes, is almost to be in a south European provincial capital. Le Monde, the London Financial Times, the Herald Tribune and Newsweek sit among Islamic journals in every kiosk. White-gloved policemen direct traffic at the busiest intersections. International calls can be made from a pay phone with disconcerting ease.

Yet the older culture still presides. In the back streets, men in robes drive donkey carts loaded with produce. Live chickens dangle by their feet in shop windows.

There are surprisingly few beggars, but at the corner sits one, hand out-stretched and chanting "All Akbar, God is great." Nearby a stout woman in white veil and robes burrows into the back seat of a taxi cab, dragging behind her bundles of groceries, two children and a veiled grandmother. The cab bounces with her exertions, then lurches away from the curb into the dense stream of traffic, missing the beggar by inches. He seems not to notice.

In the cafes on the Avenue Bourgiba an old man threads his way among the tables selling small bouquets of jasmine. "Coo-coo," he repeats, a soft call to buy. Can he not speak, or is his call the traditional call of the jasmine merchant? People buy them, hold them, twirl them absently in their fingers like worry beads, place them behind their ears. The man departs, leaving a trail of fragrance behind, like a dream in the calm darkness.

To enter the souks is to step further into the past. The medina in Tunis sucks you inward, whirlpooling toward some vague and never-found center past countless shops and gateways, through narrow passages ending in a market, a courtyard where one tree grows, a blank wall.

Deeper in the old town, where few foreigners tread, an old man with a glaucous eye lies in a wooden wheelbarrow. The flies dot his eyelids. He looks up as you pass, a sudden unhooded gaze, then releases you. Deeper and deeper you go, now half lost. Alleys turn back upon themselves, the houses turn their backs on the streets. Latticed windows and great doors hide interior courtyards and high-ceilinged rooms. Each door contains within it a second, each house is closed within itself, each individual submits to the will of Allah.

Sidi Bou Said, just north of Tunis, is one of the most effortlessly beautiful villages on the Mediterranean. Its tumbled white cube houses, smothered in bougainvillea and convulvus, cling to the slopes of a steep headland pitching down toward the harbor at Carthage. To the north the blue desert of the sea stretches toward Greece, unbroken save for a few pumice-colored islands. To the south and east the pine-covered mountains of Bou Korine float in a magnificent arrested flow of land and water. Serene, majestic, superb.

The village itself, during the day, is taken over by tourists--miraculously without harm. At night it returns to the rich, who own the villas now. As often happens, wealth followed art to Sidi Bou Said. Cervantes lived here in the 17th century, Paul Klee painted here, Andre' Gide wrote "Theseus" in a small blue and white house overlooking the bay.

It is tempting in such settings to think back to older times when the Tunisian coast was rawer, less commercial than today. Twenty years ago the shores between Hammamet and Sousse were known principally for their olives and sardines, exported to Spain and Italy and hence to America. But then Europe discovered Tunisia's coastline, and the region was transformed.

Now, in Hammamet and Sousse, busloads arrive. Chartered planes and daily 747s from Bonn or London or Paris discharge pale cargoes of vacationers, most on all-inclusive tours led by guides in red jackets and sometimes blowing whistles. In July and August, the peak months, the hotels are packed. The independent traveler is then at some disadvantage.

"You are not with the group?" the receptionist at the Hotel Fourati in Hammamet asks, visibly puzzled. He scans the register and, to his vast surprise, finds a room. The hotel is a 19th-century colonial villa, with gardens, terraces and colonades stretching toward the sea. In the lobbies, bars and restaurants, on the beach and by the pool, "the group" has taken over.

This particular week the Germans hold sway: the talk, the singing, the menus all are in German. The hotel activities board changes allegiances. What last week was billed in French or English is now in Deutsch.

Next week a new nationality will predominate, cast the place in different light, change its pace and accent. The scene is repeated up and down the coast. Only Sousse, Tunisia's third largest city, has side-stepped the one-dimensionality that heavy tourism brings. Like Tunis, Sousse is a cosmopolitian city, filled with fine restaurants and lively cafes. The beach hotels spread like wings to the north and south of the town, but the old quarter surrounding the ribat, or medieval fortress and medina, remains untouched.

It is spicy and cool in the medina and life proceeds apace. As in Tunis, separate streets centuries ago were given over to individual craftsmen and traders. One souk houses the candlemakers, another the basket weavers. Perfume-makers, carpet merchants, goldsmiths, tinsmiths, rope-makers, spice merchants, leather-workers and plumbers all have their separate bazaars. Sometimes a dozen shops will line one twisting street, all selling the same thing. Everything is on display, everthing is for sale. Craftsmen work in full view; you are invited in with a low bow or quiet "salaam." There are no signs, no glass showcases, no fixed prices. Only the men and their work.

Settling on a price for such objects is the greatest of riddles. No one knows what it will be, least of all the merchant. Prices vary with the time of day, whether the merchant is hungry and wants to go home, whether it is summer and the streets are full of tourists. There is one price for Germans, another for Americans; one price for foreigners who have been in Tunisia three weeks, a different price for those who have stayed but one.

The bargaining should last an incident-packed eternity. The more time you spend, the happier the merchant will be. "You are hard, madame. Why are you so hard?" the merchant may complain to your friend when he cannot persuade her. Yet in the end it is she he admires, not you, the soft one; she who has roused his energies and eloquence, she who made the battle without which the encounter would have lost its zest, its keenness.

Of all foreigners, Americans appeal most to Tunisians these days. The hostility the U.S. inspires in many Arabs seems wholly absent in Tunisia. It was from Tunisia, in l943, that the invasion of Europe was launched, and we are remembered: the soldiers with wide smiles and baseball cards for the children.

"You are not German?" the shopkeeper asks, abruptly frank during our bargaining.

"No. American."

"Ah, la, la, la, la . . ." a long trilling of surprise. "Very few Americans here. Germans, yes. French, yes. But Americans, no. Welcome!" And then he cuts his price by a third.

It happens again in Tunis, and once again on your return to the States. The cab driver--an elderly, turbaned man--cannot understand your French when you ask the fare to the airport. He gives you that familiar look Tunisians reserve for fretting Europeans, a melancholy but sympathetic look indicating that what Allah wills one day will come to pass. Then it dawns on him that you are leaving Tunisia for the United States. He reaches into his bag and brings out a large orange, still with a few green leaves on it, and gives it to you, not saying a word.

Some weeks later, once again at home, you find in your mailbox a card from Sousse. It is a greeting, with a picture of Sousse's moorish houses, blue and white against green cypruses, a mosque poking up in the distance. "Happy years from your frends in Susa," it reads. "Felicitations, Laribi Tarek."

Laribi sold you a scarf for your mother, gave you coffee and talked about his family and plans for the future. And you are his friend from America, whom he wishes to remember, and does.