Executives from PepsiCo, ConAgra and about 20 other major food companies, as well as Wal-Mart and advocacy groups that favor labeling, attended a meeting in January in Washington convened by the Meridian Institute, which organizes discussions of major issues. The inclusion of Wal-Mart has buoyed hopes among labeling advocates that the big food companies will shift away from tactics like those used to defeat Proposition 37 in California last fall, when corporations spent more than $40 million to oppose the labeling of genetically modified foods.

“They spent an awful lot of money in California — talk about a lack of return on investment,” said Gary Hirshberg, co-chairman of the Just Label It campaign, which advocates national labeling, and chairman of Stonyfield, an organic dairy company.

Instead of quelling the demand for labeling, the defeat of the California measure has spawned a ballot initiative in Washington State and legislative proposals in Connecticut, Vermont, New Mexico and Missouri, and a swelling consumer boycott of some organic or “natural” brands owned by major food companies.

Mr. Hirshberg, who attended the January meeting, said he knew of roughly 20 states considering labeling requirements.