Tyrone B. Hayes is an American biologist and professor of Integrative Biology at University of California, Berkeley known for his research findings concluding that the herbicide atrazine is an endocrine disruptor that demasculinizes and feminizes male frogs. He is also an advocate for critical review and regulation of pesticides and other chemicals that may cause adverse health effects. He has presented hundreds of papers, talks, and seminars on his conclusions that environmental chemical contaminants have played a role in global amphibian declines and in the health disparities that occur in minority and low income populations. His work has been contested by Syngenta, the Swiss manufacturer of atrazine and the Australian Pesticides and Veterinary Medicines Authority. It was used as the basis for the settlement of a multimillion-dollar class-action lawsuit against Syngenta.