ABSTRACT Faunal sampling was carried out in 1990–91 on three springs of a karstic system located at 15 km north-east of the city of Dijon (France) to investigate the filter effect of these outlets on the drifting population of the hypogean amphipod Niphargus virei. Results showed that, in places where limestones outcropped, the karstic spring ecotone did not modify the structure of the drifting population. In that case, faunal sampling of the springs during the floods was an effective method to study the dynamics of the population living in the aquifer. However, this method could not be applied in places where deep karstic ground-water circulated through water-saturated superficial deposits prior to emerge at the land surface. Indeed, in that case, the size frequency histograms of the drifting population of N. virei were characterised by abnormally low percentages of small size individuals and/or to a lesser extent by low percentages of large size individuals. These changes in the population structure were probably caused by two types of filters exerted by the karstic spring ecotone: 1) a mechanical filter which corresponded to the change of matrix between the deep karstified limestone aquifer and the shallow aquifer; 2) a biological filter probably due to the predation exerted by benthic invertebrates which have colonised the shallow aquifer.