Aquatic pollution by release of synthetic chemicals has become of major concern in our society. Within the last 100 years, approximately 4 million new substances have been developed, the effects of which on organisms are at least partly still. Since genotoxins ränge among the most serious threats to biota and play an important role in carcinogenesis, an amendment to the Surface Water Management Act in Germany regulates the registration of the genotoxic potential of novel synthetic chemicals. Thus, there is an urgent need for Screening of mutagens in surface and drinking waters. For such purposes, the measurement of DNA fragmentation by means of the Comet assay (single cell gelelectrophoresis, SCG) in primary cultures of fish hepatocytes and gill cells seems to be a promising system, since these cells represent eukaryotic systems provided with biotransformation capacity. Each damaged cell has the appearance of a comet with a head (undamaged DNA) and a tail (damaged DNA). In mammals, the Comet assay has been reported to be a of extraordinarily sensitive tool to study genotoxic Compounds and is considered a suitable method for biological monitoring.