The law - the first of its kind in the U.S. - bans the use of petroleum-based plastic bags by large grocery and drug chains. In addition to littering the streets, the city says, the bags are responsible for choking marine life. Instead, stores may offer paper bags or compostable plastic, a more expensive and relatively new offering that grocery stores had lobbied against.

Craig Noble, a San Francisco-based spokesperson for the Natural Resources Defense Council, which supported the ban, says that biodegradable bags are a great choice and have been used successfully in other countries. "America consumes 30 billion plastic bags and 10 billion papers ones each year," he says, which use up 14 million trees and 12 million barrels of oil. The biodegradable bags, he says, "give consumers a way out of making this false 'paper or plastic' choice."

Grocers opposed the ban, says David Heylen, a spokesman for the California Grocers Association, partly because of cost. Regular plastic bags, which don't break down in landfills, cost pennies, he says; paper bags cost 4 to 5 cents, and compostable plastic bags run from 6 to 10 cents. Eventually, those costs will have to be passed on to consumers.

More importantly, he says, "we just don't think this will work. It won't reduce the number of bags used, and we would have preferred a measure which focused on recycling bags. We feel that the people who are motivated enough to go to the trouble of composting their plastic bags are also just as likely to recycle."

For marketers eager to position themselves as environmentally aware, the city's ban on plastic raises interesting positioning questions. After all, if the law requires all stores do the green thing, will stores like Whole Foods or Wild Oats lose their environmental edge with shoppers?

"I don't think so," says John Moore, a consultant and former director of national marketing for Whole Foods, who now runs Brand Autopsy in Austin, Texas. "Consumers are smart enough to know which stores have a mission that is centered around economic sustainability, and something like a plastic bag isn't going to change that. In fact, it reinforces that some stores are proactive, and some are just reactive - consumers can tell the difference."

Of course, some stores are scurrying to get in front of the entire paper-vs.-plastic question by pushing sales of reusable cloth or fiber bags. Noble says there is no arguing that these reusable bags, as long as they're sturdy enough to be laundered after any spills, win hands down. In recent weeks, such supermarket chains as Wegman's have been more aggressively selling them.

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