Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Although the Allium cepa test has been widely used to identify potentially cytotoxic and genotoxic pollutants in aquatic environments, variable non-standardized choices have been made regarding the number of plant bulbs and roots analyzed. We propose numbers for bulbs and roots per bulb when comparing the frequencies of micronuclei, mitotic anomalies and mitotic index with this test. Roots that had been treated with aqueous solutions, such as water samples collected in August 2007 from the Paraiba do Sul River at the Brazilian cities of Trememb, and Aparecida; negative and positive controls were used for bioassays. The presence of pollutants in the river water had been presumed based on our previous cytological data and an official report by the So Paulo State Environmental Agency (Brazil) on presence of fecal contaminants (Trememb, and Aparecida) and elevated dissolved aluminium (Aparecida) in the water under study. The sampling of ten bulbs and five roots per bulb was found adequate for comparative studies to evaluate with the A. cepa test the potential damage inflicted by pollutants in aquatic environments. Furthermore, even one bulb and one root per bulb was sufficient in discerning this damage, thereby shortening the time required to attain a statistically confident comparative evaluation. However, to allow for the use of statistical programs based on the evaluation of average values, and to avoid criticism based on genetic variability, we propose that three bulbs and three roots per bulb be considered as standard sample sizes for the A. cepa test.204927935University of TaubateConselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)CNPq [-PQ-1A]