Abstract‐ In the present paper the toxicity of four biocides, endrin, thiodon, malathion and phosvel, has been measured by performing static bioassay experiments withChanna gachuaas test fish to establish the median Tolerance Limits (TLmand TL50).‐ The results of bioassay experiments indicate that the fish is harmed even at a very low concentration of these biocides. The values of TLmare 0.0076, 0.0073, 0.00695 and 0.0068 for endrin; 0.01120, 0.01088, 0.01069 and 0.01064 for thiodon; 9.20, 8.10, 7.90 and 7.60 for malathion and 32.90, 32.10, 31.65 and 31.15 mg/l for phosvel for the time interval of 24, 48, 76 and 96 hours, respectively.‐ As indicated by TLm's, endrin is most toxic and phosvel least. Thiodon and malathion came in a descending order between endrin and phosvel.‐ During bioassay studies, the effect of some variables, like temerature, pH and fish‐size, on TLm's has also been observed. They gave the following results.a) An increase in temperature increased the toxicity of all the biocides.b) An increase in pH decreased the TLmas such, making the biocides more toxic toChanna gachua.c) For larger fishes, the biocide solutions were less toxic, but the same solutions were more toxic to fishes of a smaller size.‐ The safe harmless concentrations of endrin, thiodon, malathion and phosvel are ‐ 0.00289 mg/l, 0.00337 mg/l, 1.88365 mg/l and 9.16736 mg/l,