The precautionary principle is increasingly recognized as an important tool in multilateral environmental policy making, even as its practical implications remain the subject of intense debate. Drawing on Foucault's reading of discursive politics, this paper traces the emergence and effects of a specific framing of a precautionary response to new technologies found in the 2000 Cartagena Protocol on Biosafety to the Convention on Biological Diversity. This international treaty enables states to restrict imports of specific classes of genetically modified organisms (GMOs), even if the extent of the harm they may cause remains uncertain. This particular framing of precaution in an environmental treaty is novel for its application to technologies yet to be demonstrated as harmful, and can only be understood in the context of the contentiousness of controversies over GMOs, bovine spongiform encephalopathy (BSE), and hormone-injected beef in the 1990s. At the same time, what might be termed the “Cartagena discourse of precaution” has already had productive effects on a variety of other policy fields including the regulation of persistent organic pollutants and pesticides.