The lawsuit filed in June 2001 alleges that the world's biggest retailer discriminated against women in pay and promotions. If the case is allowed to proceed with class certification, lawyers say it would become the largest civil rights lawsuit ever, covering a class of workers as large as the population of Philadelphia.

It could also have sweeping ramifications. If Wal-Mart is forced to change its pay structure, it could cause other retailers to follow. The lawsuit could cover all female employees since December 1998.

"The potential damages are huge," says Perry Binder, a legal studies professor at Georgia State University in Atlanta. "Other businesses will watch this very closely."

Wal-Mart officials say they don't expect the case to be certified as a class action. Lawyers will argue for the certification today in San Francisco, but federal Judge Martin Jenkins is not expected to make a decision immediately.

A lawsuit against Denny's in 1994 is the next-largest civil rights case, lawyers say, involving hundreds of thousands of plaintiffs in a racial discrimination claim.

For class certification to occur, lawyers for the six former and current workers claiming discrimination must show that their allegations are representative of discrimination faced by all female Wal-Mart employees in general.

"We don't think they can even come close to delivering that proof," says Sarah Clark, a spokeswoman at Wal-Mart. "Wal-Mart is a great place for women to work, and isolated complaints do not change that."

The retailer is objecting to class certification on several grounds, including the sheer size of the proposed class, according to an opposition brief filed by Wal-Mart.

Lawyers seeking class-action status say Wal-Mart's female employees are paid less than men who hold the same positions. They also say there is no objective criteria for determining who moves into management and no meaningful application process.

"Our goal is to change Wal-Mart," says Brad Seligman, one of the lawyers representing the workers.

If Wal-Mart loses, he says, "It will send a message throughout the retail industry. If Wal-Mart can be taken down, anyone can."

Cases certified as class actions often settle before they go to trial. Companies have paid millions of dollars in similar employment lawsuits involving many fewer employees. In 1997, Home Depot settled a sex bias lawsuit for $104 million for a class of more than 25,000 women.