The toxic effects of cadmium and acetic acid on the indigenous Thiobacillus thiooxidans were determined in this work. Both compounds exhibit about the same extent of toxicity toward the growth of this microorganism. The EC50s of the dose-response curves derived by means of Log-Probit analysis were found to be 200 and 210 ppm for cadmium and acetic acid, resulting in a toxic unit of 0.5 and 0.48, respectively. The toxicity of cadmium and acetic acid to this microorganism is due to their direct inhibitory effects on elemental sulfur oxidation.