The long-term security of the global food supply is contingent upon the on-farm conservation of crop genetic diversity. Without it, food crops lack the ability to evolve in the face of new pests, emerging plant diseases, and changing environmental conditions. The genetic diversity of many of humankind’s major food crops is cultivated in the field, primarily by peasant farmers of the Global South. As the widening of global markets affects the lives of these farmers in new ways, the future provisioning of crop genetic resources and, ultimately, the security of the global food supply is in doubt. In this paper I investigate how the participation of Guatemalan peasants in the market economy is related to the on-farm conservation of crop genetic diversity in three crops: maize, legumes, and squash. I find that participation in markets is not inherently detrimental to the provisioning of crop genetic resources but that without the proper protections in place market participation may unleash processes that contribute to genetic erosion over time. I conclude by sketching seven policy prescriptions that would encourage the on-farm conservation of crop genetic diversity in a way that is consistent with peasant farmers’ development objectives.