The acute and subacute toxicities of trichlorfon were evaluated in blacktetras (Gymnocorymbus ternetzi). Dipping of fishes for acute toxicity was performed for a period of 24 hrs, and TLm_24h value (median tolerance limit=LC₅₀) was 12 ppm. Severe damages were observed in various organs and among them, edema and hyperplasia of gill lamellas, and epithelial edema of renal tubules were relatively prominent. The most significant change was mild epithelial deema of renal tubules in subacute toxicity test which fishes were exposed to 1.2ppm of trichlorfon for 1 week.