Chironomids are a family belonging to the order of Diptera and the suborder of Nematocera. They mainly inhabit freshwater envi- ronments, although there are some marine and terrestrial species. The species mainly considered in the present study is Chironomus riparius. Mouthpart deformities are considered a sublethal indi- cation of exposure to toxic sediments. Mouthpart deformities result from a physiological disturbande during larval moulting. To try to understand whether other parts of the larvae are also interested by the sublethal adverse effect, nucleolus activity in the salivary gland cells was related to the incidence of defor- mities. Those larvae presenting mentum deformities has a signi- ficantly higher incidence of active nucleoli in their polytenic chromosomes than not deformed ones. Deformed larvae seem thus to be synthesising, in the salivary glands, some additional proteins that are not produced....