Chironomus piger larvae are widespread in small rivers and canals strongly polluted with domestic sewage. Despite this, almost nothing is known concerning the biology of the species under natural conditions and its role in the process of river self-purification. For two years, benthic samples were collected in the Sestra River at a site about 250 m downstream of the effluent discharge drains of a sewage treatment installation where the greatest concentrations of the larvae occurred. The number and biomass of C. piger larvae were subject to marked fluctuations, viz. 96,000-348,g00 ind m -2 and 420-1,800 g m "2. 460 degreedays (onaverage) was required for development of one generation. C. pigerhas 5 or 6 generations per year depending on the hydrometeorological conditions during the growing season. C. piger larvae play an important role in self-purification of the river. They utilize precipitating seston for food and for building their dwelling tubes. According to our calculations the amount of organic matter assimilated in the area of maximum larval concentration ranged 80-177 g wet weight m -2 day -1, and 32-71 g wet weight m -2 day -1 was mineralized.