At Looe, Cornwall (south-west England), engineering problems have resulted in the Sewage Treatment Works operating on saline rather than the typical freshwater sewage input. Saline water enters the system at all stages of the tide, except low water, mainly through the open-jointed sewers embedded in the mud of the Looe River Estuary, and causes a cyclical salinity regime within the works (Jones & Johnson, in press). A consequence of this seawater intrusion is the replacement of the freshwater macroinvertebrate community commonly associated with biological filters by two species of euryhaline amphipods. In particular, Gammarus duebeni Liljeborg is permanently established at various sites in the works including the percolating filters and humus tanks (Jones & Wigham, 1988). The ability of G. duebeni to colonise and establish a breeding population at the works is probably a reflection of its wide tolerance to several environmental factors. This species is extremely euryhaline (Forsman, 1951; Sutcliffe, 1967; Pinkster, 1970) and can withstand marked hypoxic exposure (Bulnheim, 1979; Ritz, 1980). However, in addition to fluctuations in salinity and oxygen saturation, treatment works amphipods are exposed to relatively high levels of heavy metals associated with domestic and industrial inputs (Jones & Johnson, in press). Within the humus (secondary settlement) tanks, zinc concentrations are particularly elevated compared with the adjacent estuary, although there are considerable fluctuations in zinc levels experienced by animals due to the tidally based dilution of the incoming sewage (Jones & Johnson, in press).