AbstractThe water‐hygienic aspects of the possibility of a release of microorganisms modified by genetic engineering are discussed on the basis of three examples: release of large amounts of bacterial spores into a drinking water protection zone for insect control etc., biotechnological nitrate elimination under anaerobic conditions in the drinking water treatment, protection of groundwater resources when the sewage‐engineering deep‐shaft biology is applied. In future, the biotechnology with microorganisms modified by genetic engineering (GEMs) will be increasingly applied in industry (pharmacy, foodstuff production), agriculture (pest control, plant preservation) and environmental protection (drinking water treatment, wastewater treatment, disposal site systems). Therefore, the monitoring of air, wastewater and production with respect to their content of GEMs or recombined DNA is a component of production hygiene in the sense of an environmental monitoring.