Bonamiosis was first reported in association with mass mortality of flat oysters, Ostrea edulis, in June 1979 in L'Ile Tudy, Brittany. The disease rapidly spread to all of the oyster farming areas in France but also in other European countries. The French flat oyster production, which already suffered from another protozoan disease, marteiliosis, decreased from 5500 mt in 1979 to less than 2000 mt after 1980. In 2001, about 350 French farms sell 1650 mt of flat oysters. This production mainly relies on natural spat collection which specially occurs in Quiberon bay. One third of this spat is transferred from South to North Brittany for further growth. Quiberon bay constitutes an interesting site regarding the surveillance of bonamiosis because of the simultaneous presence of spat and adult flat oysters. We analyze and present herein the long term series of data collected in this area. On the period 1980-2004 more than 30000 oysters were sampled for different purposes in Quiberon bay and were tested by histology and/or heart imprints for the detection of Bonamia ostreae. Detection frequencies presented some fluctuations including two peaks in 1994-1995 and 2000-2001. Disease frequencies recorded during the first period of this study (1980-1989) were lower than those reported later (1990-2004), probably related to the flat oyster mass mortality. Differences could be noted according to animal age and sampling season. Although the disease is enzootic in Quiberon bay since 1980, the production of flat oyster is still present in this area. One can wonder how environment and cultural practices can influence the evolution of the disease and the oyster defense mechanisms while the parasite appears to inescapably lead to the dead of the infected oysters.