We analyzed effects of peer teaching on non-science major undergraduates' knowledge, perceptions, and opinions about genetically modified (GM) crops and their use in agriculture. Undergraduates enrolled in an introductory nonmajors biology course participated in a service-learning program (SLP) in which they acted as cross-age peer teachers to high school students, teaching about the role of genetics in crop improvement through traditional breeding and GM approaches. Using pre/postassessments, we found that undergraduates' opinions shifted to favor the use of GM organisms (GMOs) in agriculture after SLP participation, rising from 46 to 97%. Perceptions about risks and benefits of GMOs also shifted from 43% stating that GMOs are harmful or suspect to no students describing GMOs in that way. Knowledge about GMOs became more accurate after SLP participation. There were significant correlations between students who had negative perceptions of GMOs and negative opinions or inaccurate knowledge about them. Students recognized the effect of peer teaching on their knowledge and perceptions, identifying the repeated peer teaching as an important factor in knowledge gain. Our results suggest students developed an informed opinion about the use of GMOs through first learning the science of genetic engineering and then teaching this information to younger students.