IT'S a sweet, languid summer evening on the Amalfi Coast of Italy. A waiter walks onto the veranda carrying a vial of red-amber liquid. At your table, he dips a twig of dried oregano into it and applies a few drops to your dish. Instantly it turns into something memorably appetizing.

The magic fluid is colatura di alici, a traditional flavoring made in two local fishing villages: Cetara, six miles west of Salerno, and Pisciotta, about 60 miles south.

To call colatura a cousin of Vietnamese nuoc mam scarcely does it justice. It's the free-run juice of salted anchovies, so it's richer and more aromatic than the typical southeast Asian fish sauce, which is brine in which fish (or fish parts) have been pickled. At first, colatura smells incredibly fishy, but a few minutes later it may strike you as meaty or winy instead. It's overflowing with the protein-type savor the Japanese call umami.

Colatura is a rare ingredient, used sparingly; "It's like adding truffle oil," says Piero Selvaggi of Valentino Restaurant in Santa Monica. Currently no Los Angeles market is selling it, nor are any of our local Italian restaurants featuring it.

But it is available online. This is one chance for the home cook to get out ahead of the pros.

The ancient Romans had a fish sauce called garum, and many people speculate that colatura may be descended from garum. Others connect it with the Cistercian monks of San Pietro di Amalfi, who were salting anchovies centuries ago.

Either explanation could be right. The fact is, colatura arises naturally from the process of salting anchovies Cetara-style. When the fish are caught in summer, the Cetaresi throw them in chestnut wood barrels, alternating layers with handfuls of salt. Then the fish are pressed down by a wooden lid weighted with rocks.

By December, the anchovies have produced a bit of fragrant amber juice. A tiny hole is poked in the bottom of the barrel and a bowl collects the colatura that drips through ("colatura" means dripping or filtration).

Evocative flavor

UNTIL the 20th century, this was exclusively a homemade product. Families would exchange bottles of their own colatura at Christmas, when it was a prominent flavoring at the meatless Christmas Eve dinner. These days, four companies make it commercially in Cetara and nearby Pellezzano. (Pisciotta's version of colatura, made in terracotta urns instead of barrels, is not available outside its locality.)

"You can use colatura anywhere you'd use salt," says Naples-born Enzo Battarra of Enzo & Angela in West Los Angeles. "Just a hint," warns Carla Capalbo, author of "The Food and Wine Guide to Naples and Campania" (Pallas Athene, London, 2006), "or it becomes unbearable."

The most common thing to do, though, is to make a salsetta by mixing a few tablespoons of extra-virgin olive oil with a clove or two of crushed garlic and a teaspoon or so of colatura. This "little sauce" most often goes on spaghetti, linguine or vermicelli (the Amalfi Coast is renowned for its artisanal pasta), but it is also used with fish.