A Poly Vinyl Chloride (PVC) Company discharges its effluents into a Provincially significant wetland that is also part of an International Joint Commission (IJC) designated Area of Concern (the Niagara River watershed). Its discharges contain a variety of compounds including organotin and vinyl chloride both of which are known carcinogens. From 1986 to 1989, a study of the frequency of chironomid (Diptera) larval mentum deformities was carried out at 15 sites in the Niagara River Watershed. The PVC Co. discharge site displayed the highest frequency of chironomid deformities (47%). By 1991 the frequency of deformities at the PVC Co. discharge site had fallen to 25%. Chironomids were absent at the site closest to the Company's discharge pipe, and occurred in relatively low numbers at sites further downstream increasing in abundance near the confluence of the discharge creek and the Welland River. Chironomids nearest the Company's discharge belonged primarily to the pollution tolerant genus, Polypedilum. This fact coupled with the high frequency of chironomid mentum deformities and the low benthic invertebrate density and diversity was interpreted as indicating an impairment within this portion of the ecosystem, as defined by the IJC.