The deliberate genetic manipulation of animals, plants and bacteria to contain novel genes (transgenes) has given rise to public fears and scientific concerns. One of the many areas of concern has been the potential for genes to move from transgenic organisms to other organisms such as bacteria. This movement of small segments of DNA (including transgenes) from one organism to another without reproduction is termed horizontal gene transfer (HGT). A key concern is that transfer of transgenes from genetically modified (GM) plants to soil bacteria could result in new biological entities with the potential to cause irreversible environmental damage. We are utilising a model soil microcosm system to investigate the frequency of transgene transfer from the genetically modified tobacco plant (Nicotiana tabacum) to the soil bacteria Acinetobacter calcoaceticus. In this system, we are testing key factors (abiotic and biotic) in the New Zealand environment for their ability to influence the transfer of the antibiotic resistance gene ‘nptII’ from GM plant DNA to the bacterial chromosome. Differences in HGT frequencies have been observed between three different New Zealand soils in a sterile microcosm system. Chemical compounds mimicking rhizosphere extracts have been shown to increase HGT events. Further analyses of factors such as temperature, moisture content and rhizosphere extracts from key New Zealand cropping and native plants are being carried out. Data generated from these experiments will used to develop a model for the spread of transgenes in the environment.