The sturdy brown paper grocery bag was a fixture. A verity. An expense.

So when the crinkly plastic grocery bag made its U.S. debut, promising grocers big savings, it rapidly took over a large share of the market.

Virtually unknown a decade ago, the petrochemical grocery bag now accounts for between 35 and 50 percent of the 30 billion bags sold annually, up from 5 percent as recently as 1982.

Grocer resistance to the new product melted in the face of enormous savings on the bags, and consumer resistance melted as shoppers became accustomed to the bags and overcame a fear of falling groceries.

The paper bag "certainly won't disappear, but I think it's going to continue to lose market share and get smaller and smaller," said Buck Williams, vice president and general manager of the bags division of Union Camp, a longtime manufacturer of paper bags that also produces the plastic counterpart.

"The general average rate of plastic versus paper is running 60 percent in favor of plastic and 40 percent in favor of paper," said Barry Sher of Giant Food Stores, which offers both kinds of bags.

Brian Dowling, a spokesman for Safeway Stores Inc., said the chain anticipates that 70 percent of its customers ultimately will be won over to plastic bags.

"There has been something of a learning curve for both employees and customers," he said.

"We find that once the employees and customers learn how to use them that customers uniformly love them."

When the product was still new, the sales staff of Mobil Chemical Co., a major bag manufacturer, pitched it by taking a single plastic grocery bag and loading it up with four six-packs of Coca-Cola. While the sales staff talked, the bag would dangle from a rod in a conference room.

Two hours later the bag would still be intact, with not a caramel-colored drop spilled on the floor, said Roger Hinds, Southwest regional general manager for Mobil's Plastic Packaging Division.

Mobil Chemical, a subsidiary of Mobil Oil Corp., and Sonoco Products Co., a manufacturer of consumer packaging, are the major manufacturers of the plastic bags, but a number of other companies also make them, including some that sell paper bags.

"The main thing with plastic is the convenience of the handle," said H. Gordon Dancy, vice president of Sonoco's polysack division, which expects to produce about six billion bags this year. The plastic bag "goes to the beach a lot, and it becomes the extra set of luggage," he said. "If you go to the airport now, you'll see a lot of them."

"Mobil in 1988 will sell more than four billion bags," said Hinds, who presides over a plant in Temple, Texas, where two new manufacturing lines are being built to supply the demand. "It's growing by double digit percentages a year," he said. Mobil also makes Hefty trash bags, other food bags and the tear-off bags that shoppers use to bag vegetables, all at the Temple plant.

For a supermarket, "one of the major costs is packaging to fill a customer's order," said Sher. According to industry officials, plastic bags moved into the market rapidly because of their cost advantage and because they take up less room than paper bags. Plastic bags cost about 2 cents each compared with 3 to 4 cents each for paper bags. In addition, each paper bag takes up as much space as six plastic bags, according to Hinds. "A supermarket lives or dies on space."

The bags are not without problems, however. Because they are not biodegradable, they have aroused concerns about their impact on the environment. Some manufacturers are experimenting with a plastic bag that would be photodegradable (that is, it would break down in sunlight), while others are promoting cornstarch bags that have some of the attributes of polyethlene bags but are biodegradable.

Manufacturers of the plastic bags note that their products can be incinerated efficiently and may also be recycled.

Still another problem is that the bags fall over in car trunks or on car seats. Manufacturers say that if the bags are properly packed with boxed products on the sides to make a rigid wall, they will stay upright. In addition, consumers are learning to put the bags on the floor or in trunk wells to keep them from falling, they said.

"Fifty years ago, when the grocer exchanged the grocery box for a bag, the customer came in and said that he wanted the box back," said Hinds. Just as customers came to accept the paper bag, so they will grow to accept its successor, he said.

Grocery and drug chains and other high-volume retailers that give away millions of bags a year will encourage their customers to choose the cheaper alternative, said Williams of Union Camp. "You're talking about businesses that historically operate on low margins. They're not going to pass up savings like that. If you're Gucci Stores you don't mind paying $1 a bag because you put $200 or $300 worth of merchandise in it. But if you're Safeway and you're putting $30 worth of food in something, it makes a big difference."