The mining of the tar sands which are included in the area to be developed by Syncrude Canada Ltd. will require the dewatering of the mine pits. This will involve the pumping of large volumes of saline groundwater. Present plans call for its eventual disposal through Ruth Lake, the Poplar River and, finally, the Athabasca River. This study was designed to determine whether groundwater from the mine area is toxic to aquatic organisms and, if so, the concentrations at which this toxicity is expressed. A variety of species, including both fish and aquatic insects, was tested to determine the range of sensitivity among aquatic animals. The resultant data can, with some qualifications, be used to estimate the maximum safe concentrations of groundwater which can be added to natural waters with minimal risk of toxic effects.