Chironomid mouthpart deformities have often been associated with sediment contamination and are, therefore, currently used to
assess sediment quality. Deformities were only occasionally induced in laboratory bioassays. Mouthpart deformities result from a
physiological disturbance during larval molting. In the past few years it has been shown that some chemicals can exert negative
eects on both vertebrates and invertebrates at the level of endocrine regulation. As insect molting is hormonally regulated, we
wanted to test the hypothesis that deformities are induced due to a hormonal disruption in the developmental process. The aim of
the present study was to test whether the endocrine disrupter, 4-n-nonylphenol (4NP), induces mouthpart deformities in chironomids.
A laboratory bioassay was performed exposing Chironomus riparius larvae to 10, 50 and 100 mg lÿ1 4NP. Survival of the
larvae was not aected by the tested concentrations, but the frequency of mentum deformities increased signi®cantly (P