Two major regulatory regimes for planting of genetically modified (GM) crops have emerged: one where the property rights for growing GM crops are mainly with the GM farmer and another where the property rights are mainly with the non-GM farmer. In this contribution, the regulatory model chosen by Canada and the United States is compared with that of the EU and its variants, analyzed from an efficiency point of view. While the general view in the literature on ex-ante regulation versus ex-post liability rules under uncertainty holds that the most efficient regulatory regime depends on the specific case under investigation, we have investigated the analytical conditions for one or the other regulatory system to be more efficient, concluding that the property rights systems are almost equivalent, so long as transaction costs are not prohibitively high and using the court system is costless. As using the court system is not cost free, however, we hold that property rights regimes where the GM farmer is not liable are preferable from a social welfare point of view.