There are many examples of organisms which, by new genetic construction or by finding themselves in new habitats, have established large populations. Some have changed ecosystems drastically, some slightly, some apparently scarcely at all as a result. In the light of these natural and unintentional experiments, protocols for the examination of proposals for genetic release can be derived. The risk of damage will be small, but the damage that could be caused is large. Because of the variety and subtlety of ecological interactions, some dangerous organisms will be passed as safe. Methods of control need to be considered at the time of release., (Copyright © 1988. Published by Elsevier Ltd.)