ABSTRACT: Leukemic malignant lymphoma was diagnosed in a black bullhead collected from Allouez Bay, in an industrialized Lake Superior estuary near Superior, Wisconsin, USA. No gross signs of thymic involvement were observed and it is believed this tumor originated in the hematopoetic tissue of the kidney. Neoplastic cells infiltrated excretory kidney, spleen, liver and gill tissue. This finding is of interest since occurrence of lymphoma has not previously been reported in this fish species. In addition, the general condition of black bullheads collected at this site was examined using a necropsy-based health assessment. Most parameters were within normal ranges. However, blood parameters and the amount of visceral fat suggested these fish may not be in optimal condition. Further work is needed to document the prevalence and cause of the neoplasm in this black bullhead population.