In the present study, cytotoxic, genotoxic and mutagenic actions of different concentrations (1, 10, 100 and 1000 μg/L) of an azo dye (BDCP – Black Dye Commercial Product) were evaluated using different cytogenetic techniques [(conventional dye, C banding, NOR banding, base-specific fluorochrome banding and fluorescent in situ hybridization (FISH)] applied to the Allium cepa test-organism. The use of conventional cytogenetic staining allowed us to determine that the azo dye induced cell death, chromosomal aberrations, nuclear alteration and micronuclei. By means of chromosome bandings and the FISH technique, it was possible to notice both cell and nucleolar alterations induced by BDCP, whose effects, in a general way, were not specific for any chromosome sites. The abnormalities observed made it possible to infer both aneugenic and clastogenic actions caused by the dye analyzed. The cytotoxic, genotoxic and mutagenic effects of BDCP were not completely eliminated, even after the recovery treatment of the A. cepa roots in water, showing a cumulative potential effect of the referred dye. Due to these results, we could infer that the BDCP may be a dangerous contaminant to the environment and, consequently, to the lives of the organisms exposed to it.