Benthic macroinvertebrates were collected at two-week intervals between 1982 May 13 and August 18 at 16 sites along an 85 km stretch of the Athabasca River between Fort McMurray and the Ells River. Samples were collected from gravel bars with a 0.l m2 cylinder sampler. All netting had a mesh size of 0.25 mm. Altogether, 348 samples were collected, containing a total of 27 229 specimens belonging to 68 taxonomic groups. The 32 species of Ephemeroptera comprised 21% of the total specimens. Chironomidae, Oligochaeta, Trichoptera, and Plecoptera comprised 53%, 18%, 2%, and 1% of the total specimens, respectively. Densities decreased between May 13 and May 28, then increased steadily to a maximum of 3 294 ind/m2 in early July, after which densities declined again. Individuals belonging to size categories >2 mm, 1 to 2 mm, 0.5 to I mm, 0.25 to 0.5 mm comprised 16%, 20%, 48%, and 16% of the total individuals collected. Average size was largest in early June. The density of invertebrates downstream from the Suncor Tar Sands Mining and Extraction Plant was 31% lower than at sites upstream from the plant. There were no site-specific differences with regard to number of taxa or Shannon-Weaver diversity. Abundance and composition of invertebrates upstream of the Suncor plant were influenced by the confluence of the Clearwater River and by the effluent from the Fort McMurray Sewage Treatment Plant.