My father, who considers himself a connoisseur of the condiment, likes to tell the story of how his mother made her own fish sauce when he was growing up in the waterside province of Thanh Hoa, south of Hanoi. Each year, when the heavy summer rains left in their wake an abundance of fish, she would purchase enough anchovies to yield a year's supply of the sauce. She would layer the fish with salt in a 50-gallon earthenware jug with a spigot at the bottom, cover it with a bamboo rack weighted with a heavy rock, then top the container with a palm leaf shaped into a cone. The jug was kept outdoors, where the tropical sun stimulated the fermentation process, and about three months later, Grandmother would siphon off the accumulated liquid. This slightly oily, richly flavored, and deeply hued substance—called nu'o'c cot (in the North) or nu'o'c nh'ĩ (in the South)—is the most prized type of fish sauce, usually reserved for dipping sauces or for special occasions. Grandmother would then make a second batch by adding water and more salt to the jug and letting the mixture ferment for another month or so. This lesser-quality liquid—called nu'o'c mam, the generic name for fish sauce—was used for everyday eating and cooking.

Today, most fish sauce is mass-produced and it is virtually impossible to purchase the unadulterated nu'o'c cot—to the regret of my father, who has never forgotten his mother's homemade versions. After leaving home and starting his own family, in fact, he used her fish sauce as a yardstick for quality and taste, seeking out small producers known for selecting the best fish and carefully combining different species to create a well-balanced sauce. But the businessman in him also smelled opportunity: One of his ventures was a plan to provide a concentrated form of fish sauce, which could be diluted with water, to the South Vietnamese army. Dad figured that one of the most comforting things a battle-weary soldier could eat was a bit of fish sauce and rice. He experimented at home, reducing whole saucepans of the liquid, and concluded that his idea was feasible. Unfortunately, the army didn't agree with him (although it later introduced a similar product).

My uncle (Bac) Nguyen Quoc Thanh didn't have such a romantic view of fish sauce. On the contrary, he saw it as a national security issue. From 1960 to 1963, my father and uncle were assigned to important posts in the South Vietnamese army and navy, respectively: Dad served as governor and chief of province of Phan Thiet, the coastal seaport and region near Ho Chi Minh City (then, of course, Saigon) that produces the majority of the country's fish sauce; Bac Thanh, meanwhile, was appointed commander and chief of district of Phuquoc (or Phu Quoc), an island off the coast near the border of Vietnam and Cambodia, renowned for making the best fish sauce in Vietnam. The waters around Phan Thiet are home to a wide variety of marine life, including mackerel, squid, and shrimp, which are blended together in the region's numerous processing plants to create a sauce with a uniquely robust taste. By contrast, Phuquoc fish sauce is made exclusively from ca co'm, a pale, nearly transparent type of anchovy common to the area. Its delicate flavor and aroma yields a lighter, more refined product.

Since Phan Thiet and Phuquoc were the prime sources for this highly nutritious staple of the Vietnamese diet, Bac Thanh points out, he and his brother had substantial responsibility. ''If the two of us didn't do our job well,'' he says, ''the entire nation would have suffered a major loss.'' (Today, Phan Thiet and Phuquoc still produce Vietnam's best fish sauces—but their names are applied to any good-quality fish sauce, whether it's made in those places or not.)

Important though it is to the Southeast Asian table, fish sauce is not exclusive to Vietnam and its neighbors. Over the centuries, fermented fish products have figured prominently in many other cuisines as well. In China they date back to the first millennium b.c., although they lost favor about two thousand years ago to fermented steamed or boiled beans and other vegetable-based condiments—precursors to both fermented soybean paste and soy sauce, which now reign supreme throughout northeastern Asia. But in Japan, fish sauce remains a key ingredient in shottsuru nabe, a famous hot-pot dish. And some Korean cooks value the convenience of fish sauce, substituting it for salted fish in kimchi and other dishes. Even the ancient Romans had a taste for it, using a thick, fermented fish sauce called liquamen or garum.

Still, Southeast Asia remains the world's largest producer and consumer of fish sauce—and transplanted Southeast Asians continue to treat it as a staple. On a recent shopping trip to an Asian market in Los Angeles, I counted over a dozen brands from Thailand, the Philippines, and Vietnam. There are slight differences in character according to origin. For example, Vietnam's nu'o'c mam tends to be sweet in flavor (a touch of sugar is usually added to tame the fishiness), while the heavier Philippine patis is designed to work with a cuisine of bold, salty, and sour flavors.

In the Philippines, another fermented seafood product competes with fish sauce. Bagoong, fermented shrimp paste, is as important as patis. Both can always be found on the kitchen table and are sometimes used in tandem. But when I asked my Philippine friend Vida about the differences between the two condiments, she said emphatically, ''Fish sauce is the big seasoning. We use it all the time. I have to have it or certain things won't taste right!'' I know exactly what she means: For me (and my family), fish sauce is worth its weight in gold.