Bonamiosis due to the intrahaemocytic protistan parasite Bonamia ostreae is a European endemic disease affecting flat oysters Ostrea edulis. After its first description in June 1979 in L'Ile Tudy, Brittany, the parasite rapidly spread to all French oyster farming areas and in other European countries through transfers of live molluscs. Bonamiosis and marteiliosis, another protozoan disease appeared in the seventies, drastically reduced the French flat oyster production from 20 000 tonnes in 1970 to less than 2000 tonnes after 1981. In 2001, about 1650 t of flat oysters were marketed in France. The production is restricted to several areas specialised in reproduction, growth and/or marketing, and depends on transfers of animals. Most important production areas of flat oysters have been surveyed regarding the presence of the parasite Bonamia ostreae since the first appearance of the disease until nowadays. This long term data series has been analysed revealing some fluctuations of detection frequencies. Differences could be noted according to animal age and sampling season. Indeed, adult oysters are usually more often detected infected than juveniles and winter or spring corresponds to the maximum recorded detection frequencies within a year. Potential impact of environmental factors including temperature and salinity has been investigated and will be discussed in order to understand the evolution of the disease. Although the disease is enzootic in France since 1980, the production of flat oyster still exists suggesting a stable host-parasite system.