The current debate on the release of genetically modified organisms to the environment must be informed by scientific data obtained from field studies. Many of the microorganisms that have potential applications outside the laboratory, especially in agriculture and horticulture, could be improved by genetic modification. Rhizobia, the bacteria that form N2-fixing symbioses with leguminous plants, have a long history of safe use as seed inoculants, their biology is relatively well known, and they represent a relevant model system. There have been several field releases of genetically modified (GM) rhizobia in the USA and Europe, which provide information on various aspects of their ecology and efficacy. This review summarises the rationale for each release, details of the methods used for monitoring, and the results. Novel properties of rhizobia did not always have the predicted effects. Most studies revealed that rhizobial numbers dropped rapidly after application to soil or seeds but then numbers stabilised for months or years. The monitoring of survival and spread of rhizobia was greatly improved by the presence of novel marker genes. Tagging of rhizobia with marker genes provided more accurate information compared to the use of conventional strains, illustrating an important application of genetic modification, for tracking bacteria in the environment.