Data from a survey on the distribution of fish species in Switzerland were used to test the hypothesis that species richness of the fish fauna is closely related with the structure, heterogeneity and complexity of the ecotonal zone in rivers . Species richness increased with increasing structural heterogeneity and variability of the ecotone in rivers wider than 2 .5 m at altitudes below 500 m . In lowland brooks and in rivers at higher altitudes other factors seemed to be more important than morphological variability : alpine rivers are more often left natural than lowland rivers and salmonid stocking is more important in these naturally species poor fish communities . The proportion of threatened species was higher if riverbed and -banks were natural, the slope was flat, the variability in depths and sediment composition was high and if the ecotonal zone was well structured . Specialized species such as nase and riffle minnow only occurred in larger rivers, whereas in small and medium-sized rivers with low morphological variability these threatened species were missing . On the other hand, ubiquitous species such as chub, roach and perch, occurred in all sizes of rivers, even if morphological variability was small . Low morphological variability and its impact on the composition of the fish community is mainly a consequence of past human interventions . Therefore river management has to take into account the ecological requirements of specialized species and increasing the variability of riverbed and banks would considerably contribute to the conservation of endangered fish species .