A little zipper has touched off a tempest in a plastic bag.

In four years, Hefty OneZip bags, which are sealed with a sliding tab, have zipped from zero to fourth place in the market for sandwich, freezer and food-storage bags, which totals $900 million in sales annually.The better mousetrap, made by Tenneco Inc., has been followed by at least one rival zipper bag, which sparked a federal patent lawsuit. And with legions of lunches and leftovers at stake, OneZip is about to launch an advertising and promotional blitz.

Beyond being a hit with brown-baggers, the zipper innovation has spread to packages for vegetables, bubble gum, even diaper wipes. And with Tenneco in talks to sell its easy-closing zipper bags to makers of everything from laundry detergent and medical waste to cigars and potato chips, consumer-products giants S.C. Johnson & Son Inc. and Clorox Co. are circling with their own bags.

The zipper innovation is one of those rare plot points in the plodding history of everyday household products. The forerunner of the newfangled wrappers, humble waxed paper, was in vogue in the 1940s. Mobil Corp. launched the plastic Baggie in the mid-1960s and later added a tuck-in flap. In 1970, Dow Chemical Co. weighed in with Ziploc, the original pinch-and-slide bag.

Today's OneZip, known technically as the "rolling-action zipper profile and slipper," was developed during the early 1990s by a Mobil team working in strict secrecy. Mobil went through numerous zipper designs before settling on the boxcar-shaped version. It snaps over dual plastic tracks and slides like a gondola on a cable.

Mobil had invested $25 million to $50 million in OneZip and demanded a premium for it when Tenneco bought Mobil's plastics business four years ago for $1.27 billion.

"We knew this was a product that had some legs," says Richard Wambold, a Tenneco executive vice president.

The reason OneZip is thriving is transparent: Americans pay up for convenience. In Ypsilanti, Mich., Faith M. Paull, who has nerve damage in her fingers, used to carry her pills in the pinch-and-slide bags. But she switched to OneZips last year as soon as she spotted them in her local grocery store. Even if the bags cost more, she says, "I'm willing to pay."