Mouthpart deformation in chironomid larvae is induced by exposure to chemical contaminants and is becoming an established bio-indicator in sediment assessment programmes. However, concentration-response relationships with causal agents have only been established occasionally and with varying success, In this laboratory study, instar II and III larvae were exposed to sub-lethal concentrations of lead, mercury and β-sitosterol. A significant deformation response was induced in the pecten with lead and mercury. Deformation frequencies of the mentum after metal exposures were not significantly different from the control. Moulting was retarded by both metals and was well correlated with mouthpart deformation. The β-sitosterol is an endocrine disruptor, which was used to test the hypothetical cause-effect relation between disruption of ecdyson functioning and chironomid deformation. In the present study, exposure to sublethal concentrations of β-sitosterol did not result in any effect on deformation or moulting. As such, the proposed hypothesis of endocrine disruptors as primary casual agents of chironomid deformation could not be substantiated. Acetone, which was used as a solvent to apply β-sitosterol caused a significant increase of mentum deformation. The ground filtration paper used as substrate seemed to induce deformities as well. Substrate contamination, acetone and (especially) inbreeding were most probably responsible for the high deformation frequencies in the control conditions. (C) 2000 Elsevier Science Ltd.