Gill necrosis is a widespread disease of common carp in Europe. Much effort has been concentrated in the investigation of its aetiology during the last 15 years. The results of a three-year research project are presented here and the role of bacteria is demonstrated. This paper shows the development of the disease and the relationship between environmental stress and bacterial invasion. Both healthy and diseased carp populations were monitored for the presence of Flexibacter columnaris. Gill necrosis was reproduced experimentally using Flexibacter columnaris isolates. Based on the bacteriological investigations, artificial infection and field monitoring, three stages of gill necrosis could be distinguished.