The Bio-Bio river, running through one of the most important hydrographic basins in Chile, presents concentrations of some chemical agents exceeding the accepted values for continental aquatic environments. The area near to the mouth of the river is highly industrialized and the industrial effluents are discharge directly into the river, most of them without any previous treatment. This river provides the principal source of drinking water for a population of more than one million inhabitants in the region. To evaluate the genotoxic effects of liquid effluents from a cellulose industry and the surface waters of the Bio-Bio river obtained near to the river mouth in the proximity of Concepción city, a short-term bio-assay with cultured Chinese hamster ovary (CHO) cells was used. The frequency of cells with chromosome aberrations in metaphase, anaphase and telophase was determined at different concentrations of the liquid samples. The results show a significant increase in chromosomal damage. The frequency of chromosomal aberrations observed both in metaphase and ana-telophase is dose-related to the concentrations of liquid samples tested. The superficial water shows a significant genotoxic effect. The scope of these results is discussed and compared to results obtained in other biological models.