In Flanders (Belgium), many fish species are declining at an alarming rate, several species have gone extinct in the last few decades, and many more risk the same fate. Water pollution and river management practices have reduced and extensively altered aquatic habitats. For many species, the number of populations as well as the number of individuals per population have decreased dramatically. Moreover, it seems very difficult for individuals to develop genetic adaptations to their polluted habitats, since adaptation through natural selection is a relatively slow process. Surprisingly, previous studies reveal that various species are capable of developing adaptations to changing environments at fast rates. Studying the occurence of genetic adaptations in natural fish populations inhabiting polluted streams can therefore be of great value to evaluate the consequences of human activities on aquatic ecosystems. This is of vital importance for setting up conservation strategies.This study aims to investigate whether populations of the gudgeon (Gobio gobio) have become genetically adapted to the heavy metals that occur in their habitats. Firstly, populations from a downstream Cd and Zn pollution gradient will be sampled by means of electrofishing. At each site, the levels of water pollution will be examined. The individuals' and populations' fitness will be compared using a number of fitness-parameters. Secondly, microsatellites, mitochondrial d-loop sequences and allozymes will be used to genetically characterise the populations. Correlations of frequencies of certain unique genotypes with the pollution gradient may be due to selection pressures of pollutants in the past. Furthermore, levels of gene flow between sample populations can be investigated. This information is needed since gene flow can affect the evolution of adaptations by causing outbreeding depressions. In a later stage, these data will be used to delineate conservation units. Since the current definition of conservation units only allows their application in undisturbed populations, our goal is to adjust for this situation, so that for all endangered populations and species conservation measures can be taken.To investigate if selection has led to new adaptive genetypes we will determine the nucleotide sequences of metallothionein (MT) genes. These proteins are believed to have a detoxifying effect on heavy metal pollution. Therefore, selection may have lead to the existence of new genotypes. Furthermore, expression of MT-genes will be studied by quantifying MT-mRNA’s in different tissues. This will be related to measured MT-concentrations in these tissues. Using this approach, genetic adaptations can be demonstrated in a direct way.The combination of both research strategies will lead to new insights in the long-term impact of industrial river pollution on aquatic ecosystems, and will help optimize future conservation efforts.