When evaluating this data, lethality (i.e. death of the organism) is the symptom being observed. Acutely lethal refers to a sample killing 50% of a population after 96 hours exposure. 1. The dike filter drainage was found to be acutely toxic to both rainbow trout and brook sticklebacks. The results of the bioassays on rainbow trout confirmed those results of S. Hrudey of the Environmental Protection Service, Environment Canada. 2. At the point of entry of the effluent, the sample was acutely lethal to rainbow trout and showed a slight degree of lethality to brook sticklebacks. 3. The downstream samples were not lethal to brook sticklebacks and demonstrated a slight degree of lethality to rainbow trout 10 - 15 feet downstream. 4. The downstream sample taken from the boat dock was not lethal to sticklebacks or rainbow trout. 5. When considering the bioassay data and excluding the point of entry of the effluent into the river, the G.C.O.S. dike filter drainage does not present a significant lethality problem to fish. 6. Acute lethality in the dike filter drainage is probably due to a combination of chemicals and not due to the action of one individual component. Ammonia and surfactants would be two chemicals making major contributions. 7. The lethal action of the interface, i.e., point of entry, is also due to the combination of the chemicals and the effect is reduced due to dilution with river water.