The waters raced into Denise Mitchell's home so quickly that she was forced to flee with nothing but what she was wearing.

After Mitchell and 10 members of her extended family took refuge in the convention center downtown, she was grateful to be on dry land but quickly realized they had another problem: Aid workers were giving out water and food, but there wasn't anything for her infant niece. The only thing Mitchell had that she thought might be of value was the silver necklace she was wearing around her neck, so she wandered around the convention center for hours trying to trade it for milk.

There were no takers. The last thing anyone wanted was something whose sole value was that it looked good.

In the most devastated areas in and around New Orleans, the lack of electricity, flooding and shortage of food is shaping an emergency economy that is markedly different from the rest of the country's -- one that centers on bartering and one whose value is measured by what one might need, not want, in a post-apocalyptic world.

"It's not about the things you enjoy and that are beautiful," said Mitchell, 36, a computer-programming student. "It's about things that'll help you survive."

For the tens of thousands of people displaced by Hurricane Katrina, the split-second decisions made as they left their homes about what to take and what to leave have gained meaning beyond what they expected. As many of the refugees were moved from filthy, overheated, crime-ridden places such as the Louisiana Superdome and the convention center to decent shelters or from hotels and relatives' houses to more permanent homes, the value of each item continued to shift.

"Green money," as some people in the city and surrounding areas now refer to cash, in some places has become nearly obsolete. In the convention center, the only person who wanted money was the guy who saved a barbeque grill and was trying to charge people to heat up their meals, which many laughed at given the boiling temperatures.

Meanwhile, things people never thought much about had suddenly became fabulously valuable. A clean pillow and blanket, or even a sheet, could buy you practically anything, be it diabetes medicine or a piece of meat. Office chairs with wheels, to ferry around the ill and weak, were worth more than DVD players and laptop computers.

Cigarettes and liquor were initially valuable commodities, a comfort in miserable conditions. But after looters flooded the market, there was so much of the stuff that people started giving it away.

In the minutes or seconds that many had to grab their possessions, some were focused squarely on the practical. Some say they regret leaving behind mementos and keepsakes, now lost forever in the mess of sludge and unspeakable things in the waters that run through the city. Others have the opposite view -- that they should have focused more on supplies for survival.

Butch Upchurch, 38, and his wife, Karen, 41, who work for a cruise company, had about five minutes to leave their home in Arabi on the western side of New Orleans. They grabbed their bank statements, insurance papers and some water and food. Upchurch's mother, his son and two of his nephews squeezed into the Chrysler 300. There was room for one of their dogs, but pit bull CoCo, the larger one, didn't fit. They kissed her goodbye and prayed she would survive.

Now that Upchurch has had time to think about it, he wishes he could have saved her, he wishes he could have brought his wedding pictures -- they were left lying on the bed as the family rushed to gather the other things -- and he wishes he had made room for the soapbox derby he has had since 1978.

"It's a part of my past that I'll never get back," said Upchurch, who is now staying at a relative's home near Jackson, Miss.

Patricia Green, 57, a librarian, and her family took four black plastic trash bags and filled them with items such as peanut butter, crackers, toilet paper and water and pushed them along in the flooding as they made their way to the highway to find help. She now wishes she had had time to get her old pictures and some toys for her grandchildren.

Others saved what seemed most precious in their old lives.

Amanda Batista, 20, an assistant manager at a photo studio, took her high school diploma card noting that she had graduated Aug. 29, 2003, from John F. Kennedy High School. Linda Harold, 49, a preschool teacher, brought all her family albums and not much else in a large black rolling suitcase.

Ariel Turner, 14, a junior high school student, packed her hot-pink duffel bag with jewelry, ribbons, her furry white gorilla stuffed animal and, in a last-minute inspiration, a stick of deodorant. Her mother wishes Turner had brought some more essential items such as clothes, but Turner said that the items comforted her through her five-day stay at the convention center and that she wouldn't trade them for anything else.

Bartering has been most acute at places such as the convention center, where food supplies have been limited and where the law-enforcement presence was increased only on Friday, leaving its temporary residents to govern themselves.

The ones in the most difficult situations were people such as Mitchell who had nothing but the shirt on their back.

But Mitchell was resourceful, and it took her only a few hours to figure out the rules of the new economy. After scrounging in some trash bins, she found T-shirts someone had dumped. She went to people who were evacuated in their pajamas or were wearing wet clothes and bartered until she got not only milk, but also a bag of Huggies diapers.

As people were asked to reduce their possessions before boarding helicopters and buses to leave the convention center, massive discard piles began to accumulate on the sidewalks: high-heeled sandals, a rolling laptop case, a green ballpoint pen, an unopened bottle of whiskey, an "I Learn Through Music" sing-along toy, and a water-damaged copy of the King James Version of the Holy Bible.

Among the few who say they have no regrets about what they took is Robert Thomas, 46, a day laborer who used to cut the grass in a city park and who goes by the nickname "Q." He had only a few seconds to grab what he could as water rushed into his apartment, but he knew exactly what he wanted: his mountain bike and a baseball bat.

His decision was prescient: As chaos descended on the convention center this week, with reports of rapes, kidnappings and murders, he knew he had made the right choice. The bike helped him "to get around fast," Thomas said. And the bat to make sure nobody can "mess with me."