Chefs call it their "go-to" ingredient, the one they reach for when a dish is blah or bland or just isn't coming together. Fish sauce, the pungent, tea-colored liquid that perfumes the cooking of Southeast Asia, has infiltrated Western kitchens.

"If something doesn't taste quite right," says Donald Dellis, chef-owner of Grasshopper in Oakland, "fish sauce is often the ingredient it needs."

It's hardly surprising that the condiment appeals to Bay Area chefs with a penchant for Asian flavors, like Hiro Sone at Terra in St. Helena, James McDevitt at Budo in Napa and Arnold Wong of Bacar and Eos in San Francisco. But more telling is fish sauce's presence today in kitchens with no overt East- West bent -- such as Gary Danko and Farallon in San Francisco -- and in dishes as definitively Western as bouillabaisse and Caesar salad.

"It adds that depth that you can't get from anything else," says Wong, who puts fish sauce in his lobster stock at Bacar. Dellis adds it to braised duck and to other braises that need more complexity. At Farallon, chef de cuisine Parke Ulrich uses it to finish sauces and to accent vinaigrettes, such as a grapefruit vinaigrette for scallop carpaccio.

"It's like liquid anchovy," says Danko, who uses it in a lobster salad and in a soup with coconut milk and smoked duck. "It's very high in umami and helps make bland things taste fuller, more round." Umami, often identified as a savory taste, derives from an amino acid found in high-protein foods.

Known as nuoc mam in Vietnam, nam pla in Thailand and patis in the Philippines, fish sauce smells like a cross between ripe Limburger and overripe sardines. On the tongue, it can be almost painfully salty, but it is rarely used straight. More often, it's diluted with water and balanced with lime juice, chiles and sugar to make a Thai or Vietnamese dipping sauce, or splashed into a meat marinade or curry, where it contributes an agreeable background note.

In Thailand, Vietnam, Burma, Laos, Cambodia and the Philippines, fish sauce is never far from a cook's hand. Didier Corlou, the French chef at the Sofitel Metropole hotel in Hanoi and author of a charming booklet about nuoc mam, writes that for him it is almost a drug, a seductive seasoning that he relies on every day.

In the West, fish sauce has been traced to classical Roman times, when it was known as garum. Pliny described it in the first century A.D., and archaeologists have found amphorae used for its production. Alan Davidson, the food scholar, writes in "The Oxford Companion to Food" (Oxford University Press, 1999) that the garum amphorae found at Pompeii still reeked of the stuff.

When fish sauce arrived on Southeast Asian tables remains a mystery, but its invention was inevitable. In a region blessed with abundant fish but, even today, lacking refrigeration, people naturally figured out how to preserve this valued protein source.

The condiment can be made with a variety of fresh and saltwater fish, but most versions rely on anchovies, which are plentiful and low in value. The whole fresh fish are packed in sea salt in wooden barrels, earthenware jars or concrete bins, and allowed to ferment for six months to a year, or even longer, before the resulting liquid is drawn off, filtered and bottled.

This first extraction is the highest in protein, the best flavored and the most valued, but most producers will re-cover the fish with brine to produce a weaker second or even a third extraction, analogous to re-steeping tea leaves. Although a few manufacturers bottle these different grades separately, others will blend them, diluting their best product in the interest of more volume.

According to Kasma Loha-Unchit, an Oakland cooking teacher who leads frequent culinary tours to Thailand, none of the fish sauce available in the United States is pure first extraction. It has all been diluted to be competitively priced.

In Vietnam, the first extraction is called nuoc mam nhi, a phrase that appears on the label, and some cooks reserve this premium product for raw uses, such as vinaigrettes, dipping sauces or final off-the-heat seasoning. For marinades or cooked dishes, they use the less expensive blends. At least two widely distributed imported brands, Viet Huong (also known as Three Crabs) and Phu Quoc, are labeled as nuoc mam nhi, but Loha-Unchit doubts that claim.

Certainly fish sauce labels raise more questions than they answer. Why does Squid Brand fish sauce have a picture of squid on the label but contain no squid? Why do manufacturers put a Vietnamese name, such as Viet Huong or Phu Quoc (the name of a Vietnamese island renowned for its fish sauce), on the bottle when the product is made in Thailand?

Although traditional, premium fish sauce contains nothing but fish and salt, all the brands available here include sugar. Some also contain hydrolyzed wheat protein (a flavor enhancer similar to monosodium glutamate) or preservatives such as sodium benzoate. Loha-Unchit says if a sauce contains hydrolyzed wheat protein chances are it was made in an accelerated fashion, not by the slow fermentation process that produces flavor naturally.

Most of the fish sauce on Bay Area shelves comes from Thailand. Vietnamese fish sauce is a rarity, although Sacramento restaurateur Mai Pham, Chronicle contributor and author of "Pleasures of the Vietnamese Table" (HarperCollins, 2001), says she sees it occasionally.

Le Chung, whose Viet Huong Fish Sauce Co. in San Francisco distributes the Three Crabs brand, says that his company has been unable to find Vietnamese fish sauce suitable to export. A skeptic, especially one who has sampled Vietnam's perfectly tasty fish sauce on its home turf, might suspect that the problem is due to price or logistics rather than quality. Charles Phan, the Vietnamese-born chef-owner of Slanted Door in San Francisco, says that Vietnamese nuoc mam is more fragrant and less salty than the Thai brands sold here, but it oxidizes quickly. The bottles he has brought back from Vietnam have turned dark and changed character rapidly.

Even Thai and Filipino brands, despite their high salt content, will darken and develop off flavors over time, eventually resembling soy sauce in appearance. Fish sauce doesn't need to be refrigerated, but it should be kept away from light and discarded if it shows signs of oxidation. At $2 to $3, a fresh bottle is a modest and worthwhile expense.

Although to some a 24-ounce bottle of fish sauce may look like a lifetime supply, it is easy to develop an addiction. Any place anchovies go, fish sauce can go, too, and more easily -- in a vinaigrette, in a tomato sauce for pasta, or on broccoli rabe or other cooked leafy greens.

"I use it in my Caesar salad in place of anchovy," Pham says. "Anchovy is a little more fishy, in my opinion. Fish sauce is more subtle, more of a caramelized flavor."

Wong adds it to bouillabaisse at Bacar and to an heirloom-tomato relish for fried calamari at Eos. Dellis adds a splash to his fried rice and to a marinade for grilled five-spice chicken. Many chefs mention fish sauce's affinity for beef and use it to season a steak before or after cooking, akin to anointing the meat with Worcestershire sauce or anchovy butter. McDevitt makes a dipping sauce for whole roast fish by marinating sliced chiles overnight in fish sauce, replicating a simple condiment found all over Vietnam.

Slanted Door customers can hardly avoid nuoc mam; Phan estimates it's in 85 to 90 percent of his dishes. It's the "secret ingredient" of his cooking, he says, and it would be impossible to replace.

Not long ago, Phan developed a claypot chicken recipe for a major frozen- food company. Everyone loved the dish, he says, but the marketing people wanted to eliminate the fish sauce.

"They said, 'Let us figure out how to make this,' " Phan recalls. "So they send in the lab-coat people, and they can't figure it out. They figured they could use caramel or 50 jars of fragrance, but they can't create that taste."

It may taste like heaven, but it smells like hell, even to a fish sauce enthusiast. Vietnam Airlines, the national carrier, won't allow fish sauce on board. A partially used bottle transported in a car invariably leaks, Loha- Unchit says, leaving a long-lasting souvenir. Even at home, keep a grip on the bottle. If it breaks, the Vietnamese say, bad luck awaits.