Since 1984, the fishery for the velvet swimming crab Necora puber in Mor-Braz (south Brittany, France) has declined significantly and the frequency of dying crabs found in fishery trawls and crabpots has increased. The etiology of these mortalities was investigated using histologica] examination of diseased crabs. A dinoflagellate parasite was found in association with pathogenic lesions of the hepatopancreas, gonads and muscles. Based on morphological features, the dinoflagellate was tentatively identified as Hematodinium sp. The presence of tissue pathology, fluctuations in crab mortality and the presence of the dinoflagellates led to the conclusion that these dinoflagellates may be the cause of the mass crab mortalities at Mor-Braz.