In 1993, sea scallop, Placopecten magellanicus, populations from the Jacques Cartier Bay on the North Shore of the Gulf of St-Lawrence of Canada, experienced masse mortality episodes with mortality rates reaching 80%. Both natural beds and aquaculture stocks were affected. Experiments were undertaken to determine if an unknown or unidentified pathogen could be responsible for the disease and if that disease was transmissible. Three experiments, in which scallops were inoculated with homogenate extracted from moribund and healthy scallop tissue, were carried followed by histopathological analysis. The results demonstrated that the scallops from the Lower North Shore were more vulnerable to external factors such as manipulations and injections than the scallops from other regions, like Magdalen Islands and Gaspésie. The histopathological analysis did not permit to identify the causal pathogenic agent but our results suggested transmission of infection. The exact cause of the mass mortality episodes remains unknown to date, but it could be caused by a combination of multiple factors including geographic distribution and inbreeding.