Chironomus riparius (Diptera) larvae were added to laboratory microcosms containing defaunated sediments sampled at 2 NO3- polluted field sites. Following a 3 wk incubation, the larval influence on the sedimentary nitrogen conversions was studied using microsensors (O-2, NO3-, NH4+) and microbial bulk parameters (microbial biomass, community respiration). At the sediment surface the chironomid larvae fed on particles (deposit-feeding layer), while in the subsurface zone the larvae moved through the sediment and ventilated transient or permanent burrows (ventilation layer). In the deposit-feeding layer of the chirononaids, NO3- production and NH4+ consumption were lower and microbial biomass decreased. In the ventilation layer of the chironomids, NO3- and NH4+ conversion maxima were shifted downwards, and both microbial biomass and community respiration were increased, The observed changes in the vertical stratigraphy of the benthic microbial community were ascribed to the depth-specific larval behaviour as: (1) particle ingestion and removal of adhering microorganisms in the deposit-feeding layer; and (2) stimulation of subsurface microorganisms due to an increased supply of O-2 and NO3- along with larval ventilation.