Serious diseases have plagued the North American oyster industry for at least 75 years. These include Malpeque Bay disease, M.S.X. disease, perkinsiosis, nocardiosis, oyster velar virus disease and others. Some of these diseases have proven intractable since they are caused by infectious agents which have not yet been transmitted in the laboratory or which cannot yet be cultivated outside of the host animal. The increased importance of aquaculture of these species and the advent of modern molecular technologies in biology have provided both the impetus and the means to now advance the science of molluscan pathology.