A very minor, instantaneous spillage of oil sands tailings sludge was introduced experimentally to a small part of a river in northern Alberta, Canada. A 60% reduction in the standing stock of benthic invertebrates occurred throughout a four-week period in the immediate area of the spillage. Sensitive indicator organisms decreased in abundance over an area of at least 30 m downstream of the spillage. The tailings sludge contained fine silt, heavy, sticky oils and heavy metals, all of which would have a deleterious effect on stream benthos. The fine silt mixed with the sticky oils of the sludge probably constituted the principal hazard to aquatic communities. It is recommended that appropriate measures should be established so as to prevent the addition of oil sands tailings sludge, by either accident or design, to lakes or rivers.