Acute toxicity (96h-LC 50) of potassium permanganate (PM) was determined for Caspian kutum Rutilus frisii kutum at two size classes (1 and 3 g). Static renewal method was used. Fish were exposed to different concentrations of PM and mortality was recorded thereafter, until 96 h. Exposed fish showed behavioral stress indicators. 96h-LC 50 was calculated to be 3.204 (3.147-3.260) mg l-1 for small fish and 3.460 (3.393-3.528) mg l-1 for large fish. Safe concentration for small and large fish was

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... lculated to be 0.003-0.320 and 0.003-0.342 mg l-1 , respectively. No Observed Effect Concentration (NOEC) and Lowest Observed Effect Concentration (LOEC) were 2.6 and 2.8 mg l-1 for small and 2.8 and 3.0 mg l-1 for large fish. Results indicated that Caspian kutum is susceptible to PM toxicity; although less than many tested species. Likewise, toxicity of PM significantly decreases with increase in fish weight. Higher tolerance in large fish compared to small fish might be as a result of increased stress resistance and higher quality of these fish.