Operationalizing the Cartagena Protocol on Biosafety will require resolving disputes about the meaning of the term 'precautionary approach' in the treaty text. Although the terms precautionary approach and precautionary principle have been referred to in the regulation of transgenic plants for nearly a decade, no customary expectation of what actions either requires has developed. If specific obligations for regulators, regulated entities, or both are not established, compliance will be impossible. This essay examines various interpretations of the precautionary principle, discusses their shortcomings, and suggests a way to rethink the regulation of transgenic plants that focuses on genuine uncertainty. Transgenic plants with familiar phenotypes should be subject to considerably less regulatory scrutiny than those whose risks are genuinely unknown, or known to pose heightened risk.