Lack of enforced regulations on the exchange of ballast water from oceangoing ships has led to its indiscriminate discharge in the environment. In this study, the cytotoxic and genotoxic effects of ballast water were investigated using the Allium cepa L. test. Bulbs with roots of Allium cepa L. were treated with different concentrations (0.5%, 1%, 5%, and 10%) of ballast water, and after 48 h the root tips were processed for cytological studies by the aceto-orcein squash procedure. All concentrations induced a number of chromosomal aberrations in the root tip cells. The mitotic index decreased significantly (p < 0.05) with increasing concentration. The cytotoxic effects showed strong concentrationdependent root growth inhibition with an EC 50 value of 15% after 72 h.Keywords: Allium cepa L; ballast water; chromosome aberrations; cytotoxicity; genotoxicity IntroductionOceanic transport accounts for more than 80% of total global shipments of commercial cargo. In addition to "intended" cargo, over 12 billion tonnes of ballast water are moved across vast coastal and oceanic domains annually, and are indiscriminately discharged into the environment, due to lack of enforced regulations.Ballast water is often sea water taken on board for stability in one country, and discharged before loading. It is a heavy material placed in the hold of a ship to gain stability. It is one of the primary vectors responsible for the global transport of vegetative and resting stages of aquatic microorganisms as well as for potentially pathogenic bacteria (Carlton and Geller 1993;Ruiz et al. 2000). Control of discharge of ballast water is thus an international issue, handled by the International Maritime Organization (IMO). Internationally, there has been slow but steady progress towards international legally binding controls on ballast water.In Nigeria, ballast water is indiscriminately discharged into the environment, due to lack of enforced IMO regulations. Discharge of ballast water from oil tankers could constitute a significant portion of the oil and other impurities introduced into the environment each year, with consequences for both animal and plant life. Results obtained for the physicochemical parameters for ballast water collected from a multipurpose vessel M/V Thelka in an earlier study by Duru and Okieimen (2011) revealed that the ballast water was basic with an appreciably high concentration of chemical oxygen demand (COD) and high electrical conductivity, heavy metals and total dissolved solids (Table 1).Currently, there is no available literature on the genotoxicity of ballast water discharged from merchant ships in Nigeria in plant systems. The Allium test has been accepted as an excellent first-tier indicator for safety evaluation of cyto-genotoxicity monitoring of chemicals, drinking water, wastewaters and complex mixtures (Fiskesjö 1985). Recent studies using the Allium cepa test include genotoxicity testing of phenoxy herbicide (Yildiz and Evrim (Olorunfemi et al. 2011a(Olorunfemi et al. , 2011b. The objective of th...