The purpose of this study was to determine the acute and subacute toxicity of bitumen to rainbow trout. The bitumen was collected along the banks of the Athabasca River downstream of Fort McMurray during 1984. Prior to experimentation, the bitumen was fractionated in water at 35°C, using an ultrasonic shaker. This procedure yielded three fractions: whole bitumen, extractable bitumen and residual bitumen. In order to assess the potential interaction of the fractions with the receiving waters (Athabasca River), three additional fractions were also generated: whole Athabasca River water, suspended solids (Athabasca River), dissolved fraction (Athabasca River). Acute toxicity of the six fractions was determined over a 96h period at concentrations of 1, 10, 50, 75 and 100 mg L-1. Subacute toxicity was determined by initially exposing the fish to the various fractions at 100 mg L-1 for 96 hours. For the next 24h, the tanks were flushed with dechlorinated municipal water, and the fish were left in this water for another 96h. At the end of the first 96h, and then at the end of the experiment, fish were euthanized and submitted for necropsy. Based on these studies, it can be concluded that: 1. The 96h LC50 of all six fractions was greater than 100 mg L-1. The fractions were considered not acutely toxic to fish. 2. In the acute studies, no significant histopathological changes were seen in fish exposed to the different fractions. 3. No histopathological changes indicative of toxicity were found in fish exposed to sublethal concentrations of the fractions. In addition, blood analysis data (electrolytes, pH, blood gases, enzymes and other biochemical parameters) were similar between principals and controls. 4. Overall, it was concluded that bitumen was not acutely toxic to fish under the dosages and conditions in which this study was conducted.