The roots of onion (Allium cepa ) stand out for having cells with large size and small number of chromosomes. These characteristics make them useful in bioassays for the measurement of a variety of cytogenetic and morphological parameters, in which they can be used as toxicity indicators of the induction and formation of micronuclei and chromosomal aberrations. Based on this background, the potential genotoxic effect of phenol concentration on cells of A. cepa roots was investigated either in terms of induced aberrations or micronuclei formation. The results demonstrated that the higher the concentration of phenol, the higher the incidence of abnormalities, thus confirming the genotoxicity of this pollutant.Universidade Estadual Paulista Júlio de Mesquita Filho, Faculdade de Ciências e Letras de Assis, Assis, Avenida Dom Antonio, 2100, Vila Universitária, CEP 19806-900, SP, BrasilUniversidade Estadual Paulista Júlio de Mesquita Filho, Faculdade de Ciências e Letras de Assis, Assis, Avenida Dom Antonio, 2100, Vila Universitária, CEP 19806-900, SP, BrasilDepartment of Pharmaceutical and Biochemical Technology, Universidade de Sao Paulo, Sao Paulo, SP, BrasilDepartment of Civil, Chemical and Environmental Engineering, University of Genoa, Genova, Italydepartment of Biology, Universidade Estadual Paulista, Assis, SP, BrasilDepartment of Earth, Environment and Life Sciences, University of Genoa, Genova, Italy