The ecological risks of genetically modified crops are of greatest concern when there are no inherent barriers to the spread of transgenes through sexual reproduction. This is most likely when transgenes can spread to weedy species through hybridization, or when the crop species itself exists in weedy forms1. If the potential recipient of a transgene is a highly selfing species, such as Arabidopsis thaliana, this risk is often considered negligible2. Here, however, we report results of a field experiment in which transgenic A. thaliana showed a dramatically increased ability to donate pollen to nearby wild-type mothers compared with A. thaliana mutants expressing the same mutant allele as the transgenic plants.