To study the genotoxic activity of Decemtione (Imidan), this substance was subjected to a series of tests. After preliminary cytotoxicity testing, the capacity of Decemtione to damage human DNA was determined by alkaline elution of DNA and DNA unwinding. Both tests gave positive results, suggesting that Decemtione was able to induce single-strand breaks in DNA. This capacity was higher in the absence and lower in the presence of the S9 fraction. The potential mutagenicity of Decemtione was followed on the basis of its ability to induce resistance to 6-thioguanine in V79 hamster cells. Unlike the induction of single-strand breaks, Decemtione showed, in the absence of the metabolic activation system, a very weak mutagenic effect, which was, however, significantly higher in the presence of the S9 fraction. The ability of the substance to transform diploid cells under in vitro conditions was followed on the basis of morphological transformation of Syrian hamster embryo cells. The results showed that Decemtione, like positive carcinogenes, induced a significant elevation in morphologically transformed colonies of embryo cells. The results suggest a carcinogenic potential of this organophosphate insecticide.