Surveillance and studies performed by The Norwegian Institute of Marine Research and the Norwegian Veterinary Institute has never revealed notifiable diseases in Norwegian populations of European flat oysters, Ostrea edulis. In 2006, microcells resembling the oyster parasite Bonamia sp. were observed during histopathological examination of tissue specimens of flat oysters, Ostrea edulis from the Arendal area, southern Norway. The cells were however not interprated as B. ostreae and PCR-analysis of samples from this oyster population were negative. In 2008, the EU reference laboratory (EURL) received samples from the Norwegian Veterinary Institute and reported one Bonamia sp. in a haemocyte from one oyster, based on microscopy. By real-time PCR, positive results were obtained from two oysters in one triplicate sample. Sequencing of the PCR products gave 100% identity with B. ostreae. After this diagnose, both the Norwegian Veterinary Institute and The Institute of Marine Research have monitored the population. The results are briefly reported here. The observed microcells were found in most samples since the sampling at the site was initiated, always at a low prevalence and intensity. No inflammation, pathology or reductions of the oyster's condition have been associated with the observation. The population appears healthy, with a normal reproductive cycle pattern. Several year classes have been present throughout the study period. Since 2009, more than 3 000 oysters have been examined. Bonamia sp. has never been detected, and all PCR assays have been negative.