Epizootics of a malignant blood cancer (lymphosarcoma) affects feral populations of northern pike and muskellunge. Overall frequencies of occurrence of the disease in northern pike and muskellunge as high as 20.9% and 16.0%, respectively, were found. The disease in feral muskellunge caused high mortalities while in northern pike spontaneous regressions are common. The disease is transmitted precutaneously during the act of spawning. The disease does not seem to be transmitted to progency via the egg. These species should be stocked as eggs or fry, not as adults, if the spread of lymphosarcoma is to be restricted.