Abstract Samples of stormwater runoff from nine sawmills in British Columbia, Canada, were tested for acute toxicity with juvenile rainbow trout over a 23-month period. Forty-two of the 58 samples tested exhibited toxicity. Causes of toxicity were investigated using toxicity identification evaluation techniques. Toxicity was attributed to divalent cations, particularly zinc, in 32 of the samples. The low hardness associated with most of the samples increased the potential for metal toxicity. For example, the LC50 of zinc was 14 μg/L at a hardness of 5 mg/L. Toxicity in the remaining samples was largely attributed to tannins and lignins and was associated with areas of bulk log handling. No evidence was found to indicate that antisapstain chemicals applied to freshly cut wood contributed to toxicity.