In the present study, the in vivo micronucleus (MN) test in fish erythrocytes was used to evaluate the genotoxic potentials of water samples collected from four different sites along the Nilufer Stream which empties into the Marmara Sea on the north-west of Turkey. Nilufer Stream receives discharges of industrial and domestic wastes resulting from industrialization and urbanization activities in Bursa city. Nile tilapias (Oreochromis niloticus) were exposed to collected water samples under laboratory conditions for 3 and 6 days. Micronuclei analyses were carried out in peripheral blood erythrocytes. In addition to micronuclei, other nuclear abnormalities (NAs) such as bi-nucleated cells and binuclei with nucleoplasmic bridge and cells with blebbed, notched and lobbed nuclei, were assessed in the erythrocytes. Chemical analyses were also carried out in the water samples to assess the presence of major pollutants. MN and NA frequencies were significantly elevated in fishes exposed to water from polluted areas compared to those exposed to clean water sample. The results of this study indicate that Nilufer Stream is contaminated with potential genotoxic chemicals and the genotoxicity is possibly related with the industrial, agricultural and domestic activities.