Pathological changes associated with chronic exposure of channel catfish to sublethal concentrations of environmental nitrite is described. Other than mild methemoglobinemia (<15% of total hemoglobin), no lesions were evident in fish exposed to NO2:C1 molar ratios of <0.12. Fish exposed to NO 2:C1 molar ratios of 0.25 or greater developed a moderate hemolytic anemia after one to two weeks of exposure. Mild hemosiderosis in hematopietic tissues and increase in immature circulating erthrocytes were evident. Other lesions included mild intralamellar hyperplasia and hypertrophy and mild multifocal necrosis of the liver and anterior kidney. The relevance of these findings to management of nitrite toxicosis in cultured catfish is discussed.