Little is known about the impacts of transgenic plants and micro-organisms on soil biota and processes, despite the large number of field releases of transgenic crops. Recent work has shown that transgenic plants can cause changes in the soil biota (both invertebrates and micro-organisms) associated with these plants. Often the changes are transient but their impact on the soil ecosystem remains unknown. Horizontal gene transfer from genetically modified (GM) bacteria to indigenous soil bacteria has been demonstrated but movement of genes from transgenic plants, if it occurs at all, has not yet been detected in natural systems. Soil invertebrates appear to play an important role in horizontal gene transfer between bacteria in soil.