Bonamiosis due to the intrahaemocytic protistan parasite Bonamia ostreae is a European endemic disease affecting the flat oyster, Ostrea edulis. After its first description in June 1979ı in Brittany, the parasite rapidly spread to all French oyster farming areas and in other European countries through transfers of live molluscs. The parasite has been described in different ecosystems from estuaries to open sea and no clear correlations could he demonstrated between the disease development and environmental parameters like temperature or salinity. The infection can he directly transmitted by cohabitation between infected and non infected oysters. Nevertheless, the parasite life cycle, including its survival outside the host, is not completely known.1n the present study, the impact of temperature and salinity on the survival of purified parasites maintained in seawater was investigated by flow cytornetry. Purified parasites were incubated in three different 0.22 µm filtered seawater media (artificial seawater; natural seawater from Charente Maritime, France; underground salty water) and were subjected to three temperatures (4, 15 and 25°C). In other experiments, purified parasites maintained in underground salty water were subjected to a range of salinity (5, 15,20,25,30,ı 35, 40 and 45 g/l). Parasites were collected after 12, 48 hours and 1 week of incubation for flow cytometry analyses including estimation of parasite mortality and non specific esterase activities. All experiments were performed three times.ı The parasites showed a significant higher survival in underground and natural seawater compared to artificial medium. Parasite survival and non specific esterase activities were lower at 5°C than at 4°C or 15°C. High salinities (2:35 g/l) appeared to favour parasite survival and esterase activities. No significant variation of parasite survival could be identified between 12 and 48 hours of incubation at whatever temperature and salinity. After one week, parasite cells appeared too damaged to allow good result interpretation. However up to 58% of parasite survival could he observed after one week in• underground salty water at 15°C.ı Flow cytometry is an interesting technique to investigate survival of small parasites like Bonamia ostreae in different conditions. These resuIts contribute to a better understanding of the disease but need to he validated by epidemiological surveys in the field.