Malignant lymphomas have been found in pikes (Esox lucius L.) caught in the eastern part of the Baltic Sea. The frequency of the disease has increased in recent years. Three different histological types of malignant lymphoma are described in 32 pikes: poorly differentiated diffuse lymphocytic lymphoma, mixed (histiocytoid-lymphocytic) diffuse lymphoma and undifferentiated (anisomorphic) diffuse lymphoma.