The interstitial fauna living in the riverine sediments of the Danube and Piesting have been investigated in Lower Austria. The nematodes, oligochaetes and cyclopoids are the most abundant groups (they represent up to 80% of the total fauna). The harpacticoids, the insect larvae, the isopods, the amphipods, the cladocera and the limnohalacarids are poorly represented (generally under 20% of the total fauna). The absence of hydrachnellids is striking. The vertical distribution of the interstitial fauna shows for several groups i.e. limnohalacarids, ostracods, isopods, harpacticoids, that the epigean species are quantitatively better represented in the upper sediment layers instead of the hypogean species which are more abundant in the deeper layers. At one of the sites where samples were taken down to 3 m, most of the interstitial fauna was concentrated in the upper 1.50 m. The occurrence of limnohalacarids in the wells from the Danube Valley and the Piesting area shows that the repartition of this group is not restricted to the rhitrostygal zone. The distribution of the interstitial fauna in connection with the pollution of the river is discussed. High pollution inside the interstitial habitat eliminates the hypogean fauna and the epigeans disappear mainly in those areas with marked chemical reducing conditions.