SummaryFewer species and more individuals of Tubificidae were present in the soft sediments of a polluted ditch than in the relatively unpolluted Bere Stream.Changes in width frequency of natural populations ofTubifex tubifexandLimno‐drilus hoffmeisterithroughout the year were similar at both the sites investigated.In laboratory experiments larger populations ofTubifex tubifexdeveloped in fine sediments than coarse. By means of a width‐dry‐weight relationship it was possible to estimate the weight of the experimental popula