The in vivo toxicity and sublethal biochemical effects of chlorothalonil (2,4,5,6-tetrachloroisophthalonitrile) were evaluated in the channel catfish (lctalurus punctatus). The static 96hr LC50 value for chlorothalonil was 52 μg/L, indicating that chlorothalonil is highly acutely toxic to channel catfish. Sublethal chlorothalonil exposure for 6 days resulted in acute necrosis of the intestinal epithelial lining in 57% of the treated catfish. However, no treatment-related pathological lesions were observed in liver, gills, posterior kidney, brain, spleen, or heart. Catfish exposed to 13 μg/L chlorothalonil for 72 hr had increased tissue glutathione (GSH) concentrations in liver, posterior kidney and gills, suggesting a protective role of tissue GSH towards chlorothalonil exposure. © 1992.