To control the increasing pollution of streams in urban and industrial areas, ecological criteria such as indicator properties of organisms and biocoenoses are required. Hereby, the distribution and abundance of populations is determined by their genetic structure, that react in different ways to the intensity of burdening parameters.The Saar River is excessively polluted by industrial sewage, thermal wastes and chemicals. Therefore monitoring programs were established to analyze the structure of benthic populations in order to control the influence of the present pollution of the river.Genetic and popu|ation-eco|ogicat analyses of the freshwater gastropod Physa acuta Drap. indicate gradually significant differences in the allele frequencies and their heterozygoties. The results demonstrate correlated strategies in different populagions, whereby balanced allele frequency distribution patterns indicate the influence of the toxic substances.These analyses Serve for a better understanding of evolutionary processes in urban environments as well as for an objective consideration of the indicator systems by detecting changes before the extinction of a taxon.