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Giant bioengineering companies like Monsanto and DuPont are spending millions of dollars to fight a California ballot initiative aimed at requiring the labeling of genetically modified foods. That surprises no one, least of all the proponents of the law, which if approved by voters would become the first of its kind in the nation.

But the companies behind some of the biggest organic brands in the country – Kashi, Cascadian Farm, Horizon Organic – also have joined the antilabeling effort, adding millions of dollars to defeat the initiative, known as Proposition 37.

Their opposition stands in sharp contrast to smaller, independent organic companies, which generally favor labeling products that contain genetically modified organisms, or G.M.O.’s. And it has raised a consumer reaction on social media that has led some of the organic brands to try to distance themselves from their corporate parents.

“We want to be clear that Kashi has not made any contributions to oppose G.M.O. labeling,” the brand said in a statement issued late last month after its Facebook page was inundated with comments from consumers saying they would no longer buy its products because its corporate owner, the Kellogg Company, has put more than $600,000 into fighting the ballot initiative.

But as recently as last week, consumers were still peppering the sites of Horizon, owned by Dean Foods; the J. M. Smucker Company, which has a number of organic products, and Kashi with expressions of betrayal and disappointment. “It is unconscionable for you to be funding the effort to defeat Proposition 37,” one post said.