Proposition 37 would have required genetically engineered food in California to be labeled. This paper reports the results of a survey designed to determine Californians’ voting intentions prior to the vote, perceptions about the prevalence of genetically engineered foods in the United States, willingness to pay for a mandatory label, and effectiveness of advocacy advertising. Overall, Californians had inaccurate knowledge about the prevalence of genetically engineered foods, and stated they were willing to pay up to 13.8% higher food costs on average for a mandatory label. Findings suggest that the effectiveness of opposition advertising was likely a formative factor in the defeat of Proposition 37.