SUMMARY.Oligochaete worms dominate the macroinvertebrate fauna of the River Irwell, in north west England. The river is polluted by domestic and industrial wastes. Tubificids constituted 86.8% of the worm fauna, with three dominant species;Tubifex tubifex(Müller), representing 43.1% of the total oligochaetes,Limnodrilus hoffmeisteriClaparède, representing 26.8% andLimnodrilus udekcmianusClaparède, representing 16.7%. The Naididae were the next most abundant worms, representing 6.1% of the oligochaete fauna, withNais elinguisMüller comprising 5.0%.T. tubifex and L. hoffmeisteriwere abundant along the length of the river, but reached their greatest numbers in the organically polluted lower reaches at Agecroft and Salford. The density ofL. udekemianuswas low in the Irwell above the entry point of the heavily polluted River Roch, but high below this point.N. elinguiswas abundant above the Roch entry point and scarce below it.In the three dominant tubificids, the main population recruitment occurred during the summer months (April to September forT. tubifex, May to November forL. hoffmeisteriand June to October forL. udekemianus). InN. elinguis, there was massive recruitment in Ap