Proposition 37 was a California ballot measure rejected in California at the statewide election on November 6, 2012. This initiative statute would have required labeling of genetically engineered food, with some exceptions. It would have disallowed the practice of labeling genetically engineered food with the word "natural." This proposition was one of the main concerns by the organizers of the March Against Monsanto in May 2013.

Section 2 of Proposition 37, the "Statement of Purpose", reads "The purpose of this measure is to create and enforce the fundamental right of the people of California to be fully informed about whether the food they purchase and eat is genetically engineered and not misbranded as natural so that they can choose for themselves whether to purchase and eat such foods. It shall be liberally construed to fulfill this purpose".

The proposed law also includes several exceptions, such as products that are certified organic, made from animals fed or injected with genetically engineered material (but not genetically engineered themselves), processed with or containing only small amounts of genetically engineered ingredients, administered for treatment of medical conditions, sold for immediate consumption such as in a restaurant; and alcoholic beverages.

Grocery stores and other retailers would be primarily responsible for ensuring that their food products are correctly labeled. For foods that are exempt, retailers would have to provide records either directly from the provider of the product, or by receiving independent certification from third parties. Farmers, food manufacturers, and every other party in the product's supply chain would also have to maintain such records.

According to the California Attorney General, the measure would "increase annual state costs ranging from a few hundred thousand dollars to over $1 million to regulate the labeling of genetically engineered foods". It would also incur "Potential, but likely not significant, costs to state and local governments due to litigation resulting from possible violations of the requirements of this measure. Some of these costs would be supported by court filing fees that the parties involved in each legal case would be required to pay under existing law."

Proponents argue that "Proposition 37 gives us the right to know what is in the food we eat and feed to our families. It simply requires labeling of food produced using genetic engineering, so we can choose whether to buy those products or not. We have a right to know." Opponents argued that "Prop. 37 is a deceptive, deeply flawed food labeling scheme, full of special-interest exemptions and loopholes. Prop. 37 would: create new government bureaucracy costing taxpayers millions, authorize expensive shakedown lawsuits against farmers and small businesses, and increase family grocery bills by hundreds of dollars per year."

Opponents said Proposition 37 labeling requirements would increase grocery costs by as much as $400 per year based on a study by Northbridge Environmental Consultants and the non-partisan California Legislative Analyst's Office fiscal impact study.

Proponents on the other hand, said that some organic US food processors argued that the changes in labeling will have no effect on consumer costs because companies change their labeling all the time, as it is, and changing labels is a regular cost already built into the price consumers pay for products. “We, as with most manufacturers, are continually updating our packaging. It is a regular cost of doing business - a small one at that - and is already built into the price consumers pay for products,” said Arran Stephens, president and founder of Nature's Path.