Six samples of fluvial sediments have been collected: two in clean water areas, the remaining four from different very polluted reaches of the same river. About 3-4 l of mud from each sample have been put in 15 l tanks. and flushed permanently for 5 months with tap water. In the tanks with clean sediments, Secernentea (typically polluted water inhabitants nematodes; species 1-13 of Table I) have represented, in the course of the first and second trial month, merely 16% of total nematodes (fig. 1); this rate remained substantially unchanged (13%) during the third, fourth and fifth trial month. In the four tanks with polluted sediments, Secernentea represent on the whole 93% of nematocoenosis during the first two months but, proceeding with oxydation and cleaning of the sediments, this rate decreases to 8 % in the reckoning carried out from the third to the fifth month (Table I and fig. 1). Among Secernentea, Tylenchida are probably less significant as ecological indicators than other groups, since most of them are not fungal feeding and saprophilous nematodes, but primarily plant feeders.