A waterfront oyster market at Cancale, where customers can eat in or take away. The saleswomen (one shown here) are experts at opening the shells. (Susan Spano)

Cancale and Locmariaquer are dots on the Atlantic coast of France. Also places that produce my favorite food: Brittany oysters. Served raw on the half shell, with no more sauce than a squeeze of lemon, they are generally smaller than other varieties but intensely flavored, more precious than pearls to people who know their oysters.

French King Henri IV could down 20 dozen in a sitting. Diderot, Voltaire and Rousseau ate them for inspiration, as did Napoleon Bonaparte before going into battle. I could wax poetic about the appeal of the oyster but know I'd never convince people who find them abhorrent because they hate the idea of eating them raw, the vastly preferred preparation in France. Others think oysters are unhealthy because they are bottom feeders, living in one place, attached to an immobile object, siphoning gunky nutrients from seawater. Unless outrageous fortune serves you the rare bad oyster, nothing could be further from the truth; oysters contain vitamins and minerals, especially phosphorous, good for bones and teeth.

On the other hand, oyster idolatry may be a matter of instinct. "Obviously if you don't love life you can't enjoy an oyster," Eleanor Clark wrote in "The Oysters of Locmariaquer."

The point is, even today as oyster cultivators transplant varieties from place to place, all oysters are not created equal; their taste, like that of wine, depends on where they matured. So order your New Zealand Bluffs and Virginia Chincoteagues; then transcend by eating an Ostrea edulis plucked from Mont-St.-Michel Bay.

I discovered the supremacy of Brittany oysters a decade ago on a three-day trek from St.-Malo to the great gothic monastery of Mont-St.-Michel. Along the way I stopped in the village of Cancale where every waterfront restaurant had an oyster-on-the-half-shell special.

I let the first Cancale puddle on my tongue, husky with the taste of iodine and ocean floor, before releasing it down my throat, a sensory experience completely unlike ordinary eating. After I polished off the rest I sat looking over the wide, flat bay and then ordered another dozen. I could have eaten more, though perhaps not as many as the 19th century Englishman who must have set a record by consuming 12 dozen, washed down by 12 glasses of Champagne, while the clock was striking 12, according to M.F.K. Fisher in her small 1941 classic "Consider the Oyster."

Last spring I returned to Cancale as well as another celebrated oyster-producing region in the south of Brittany around the Gulf of Morbihan. I planned to visit farms, talk to cultivators and eat oysters at breakfast, lunch and dinner even though it was May, a month spelled without an R, when customs tells people to avoid oysters. Almost 50% of Brittany oysters are consumed in France in December and January, during the Christmas and New Year's holidays. But as it turns out, the R rule is an old wives' tale, born perhaps of government efforts to protect oysters by banning harvest during procreation in spring and summer.

I drove to Cancale from Paris on an overcast day, arriving at low tide. Barnacled flotsam and jetsam lay where the water had left it, and boats were keeled over in the sand, high and dry. Also exposed were acres of oyster beds, laid out like farm fields, with low tables that supported wire bags of maturing Cancales, tended by rubber-booted oystermen, some of them driving tractors.

Spanning the border between Normandy and Brittany, Mont-St.-Michel Bay is known for its tides, which raise and lower the water level as much as 45 feet a day, and made the monastery accessible only by boat before the construction of a bridge connecting it to the mainland. People still have to look sharp when the tide comes in "as fast as a galloping horse," Victor Hugo said.

This amphibious place, some shade of gray-green-blue that has not yet been named, is one of the great beauty spots of France, low on the Normandy side where farmers have reclaimed pasture land from the sea. Gorse-covered cliffs rise around Cancale, holding in the west side of the bay before rounding lofty Pointe du Grouin and heading southwest along Brittany's Emerald Coast, lined by the Grande Randonnée footpath I walked 10 years ago.

Cancale was just as I'd left it, all dour gray stone and suspicious shuttered windows. The upper town has churches, shops, a tourist office and square with a statue of women oyster workers. In the port below there was the same collection of restaurants with seafood specials, including multitiered shellfish platters with crab legs, mussels and oysters as well as tiny whelks and winkles.

This time I went to the open-air market on the pier where stalls display oysters on beds of seaweed, grouped and priced by size. A saleswomen, about as friendly as a mollusk, chose 12 No. 2s for me, stabbed them open with a short, sharp knife and then served them up on a plastic plate; the cheapest dozen I'd ever had, about $7, with a lemon, but no napkin. The ensuing debauchery, closely watched by passers-by, left juice dribbles on my shirt and salty brine on my face and hands.

I spent the night at the Manoir des Douets Fleuris, a charming little hotel a few miles inland from town. It has a handful of prettily decorated country-style rooms around a courtyard garden, a small swimming pool and an excellent restaurant that serves traditional Brittany cuisine with a contemporary flair. For my main course at dinner I ordered a salad with pillowy soft local chevre. Need I say that I started with a dozen Cancales?

I spent the next morning at St. Kerber, a working oyster farm with a museum where I saw a film about the history of oyster farming and looked around with the proprietor François-Joseph Pichot, who described the bivalve's life cycle. Those who doubt its fascination should consider some of the things I learned:

—Oysters are hermaphroditic, in other words both male and female, able to produce sperm and eggs alternately.

—When in a female mode, the mother bears up to 1.5 million eggs; only 10 in every 10 million eggs survive.

—It takes about four years for oysters to reach adulthood, when they are harvested, cleaned, sorted and shipped mostly to Paris. Pichot told me that he can't export to America because of U.S. government restrictions, not because they'd go bad before reaching North America.

—A healthy, living oyster with its shell closed tight remains good for several days, but tastes best as soon after harvesting as possible.

Nowadays oyster lovers are relatively abstemious, but in times gone by people gorged on them, especially at the start of a meal, believing that they sharpen the appetite for courses to come. In the 1st century Roman Emperor Vitellius — "the beasty Vitellius," historian Edward Gibbon called him — liked about 1,200 as an appetizer; 18th century rake Casanova routinely started dinner with 50, boosting the oyster's reputation as an aphrodisiac; English writer Samuel Johnson fed them to Hodge, his cat.