The surveillance programme was carried out in accordance with the model established in 2015, including four wild beds and one oyster farm, based on the present distribution of wild beds, and the structure of the oyster industry. Samples were collected in April/May and in October, in order to be able to detect Bonamia sp. and Marteilia sp. during the periods when the potential prevalence could be at the highest. No abnormal mortalities were observed in oyster populations during the surveillance. Bonamia ostreae / B. exitiosa or microcells resembling Bonamia spp. were not detected during the surveillance programme in 2016. There have been several reports on mortality or “disappearance” of mussels along the Norwegian coast. The reason(s) for the mortalities have not been determined. From one mortality event, at Ytre Hvaler, Østfold, South-eastern Norway, in September 2016, live, presumably moribund mussels were sent to IMR and examined. Affected mussels showed infiltration of bacteria in digestive and connective tissues. Bacteria have been isolated, cultured and frozen for subsequent analysis. No parasites were observed. Marteilia refringens was detected for the first time in mussels, Mytilus sp. collected at Bømlo, western Norway in October. The prevalence was 50% and most infected mussels exhibited a heavy infection with sporulating Marteilia cells in the digestive epithelia, and affected tissues. A plan for extended survey and a study of the Marteilia refringens in affected mussels has been initiated.