In the literature spatial coexistence of genetically modified (GM) and non-GM crops is often regarded as a technical challenge, depending on spatial pollen dispersal and cross-pollination, temporal and spatial distribution and interaction of crops, separation distances and practical measures, but the interaction between economic incentives and costs of coexistence is poorly studied. Europe is currently struggling to implement coherent coexistence regulations in all EU Member States. Since the publication of the European Commission's guidelines in 2003, some Member States have developed, and others are still developing, a diversity of ex ante regulations and ex post liability rules on the coexistence of GM and non-GM crops. In this article, our attention is drawn to ex ante regulations. More specifically, we polarize two alternative ways of regulating spatial coexistence, i.e. rigid minimum distance rules, imposed on GM crop production, versus flexible segregation measures such as buffer zones, leaving more freedom of negotiation between neighboring farmers. We conduct simulations with the software ArcView (R) on a GIS dataset of a hypothetical case of GM herbicide tolerant oilseed rape cultivation in Central France. Our findings show that rigid coexistence rules, such as large distance requirements, may impose a severe burden on GM crop production in Europe. These rules are not proportional to the farmers' basic economic incentives for coexistence and hence not consistent with the objectives of the European Commission. More alarming, we show that in densely planted areas a domino-effect may occur, a theoretical possibility ignored in the literature. This effect raises coexistence costs and even adds to the non-proportionality of rigid coexistence regulations. Instead, we show that flexible measures would be preferable since they are proportional to the economic incentives for coexistence and, hence, less counterproductive for European agriculture. (C) 2007 Elsevier B.V. All rights reserved.