Chemical products used in agriculture require detailed formulation and knowledge on their mode of action and uses. This is essential for understanding the pesticide's biological behavior and also to establish safety criteria for humans and animals. At the present time, there are numerous studies concerning physical and chemical agents present in the media which increase the incidence of tumors. In this paper, we demonstrate the mutagenic capacity of the fungicide thiram, using the microbiological Ames test, at a concentration of 1 mg/ml active ingredient in the culture medium. Higher concentrations were lethal to the microorganisms used in the test.