Opponents say labels would be costly, confusing and unnecessary.
About 80 percent of packaged foods in U.S. supermarkets contain genetically modified ingredients, the Grocery Manufacturers Association, an industry trade group, says.
Such foods have been on the market for more than 15 years and the American Medical Association says there have been no reported of negative health effects.
But some Californians say they're not willing to take that on trust.
"People used to think that smoking wasn't addictive," Pamm Larry, a leader in getting the food-labeling initiative Proposition 37 on the ballot, told Voice of America. "My understanding is, there's a lot of stuff like that."
Health problems from eating genetically modified organisms could surface years from now, she said.
"So, I just think in the meantime, people have a right to know what they're buying and eating," she said. "That's it."
Many European countries and Japan require such labels.
Opponents of the measure say the labeling requirement would raise the cost of food.
The "No on 37" campaign says it would add up to about $400 per year for each California consumer.
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