Bituminous hydrocarbons extracted from the Athabasca oil sands of north-eastern Alberta were adsorbed onto filter supports and placed at sites in the Athabasca River and its tributaries where these rivers come in contact with the oil sands formation. Colonization of the hydrocarbon surfaces at summer and winter ambient temperatures was examined by scanning and transmission electron microscopy as well as by epifluorescence microscopy of acridine orange-stained cross sections. Ruthenium red and alkaline bismuth stains visualized an association of bacteria with the hydrocarbon surface which was mediated by bacterial polysaccharides. Bacteria apparently lacking a glycocalyx were also found closely associated with the surface of the hydrophobic substrate and in channels within the substrate. A solvent precipitation and column chromatographic fractionation of the bitumen was followed by cross-tests for growth on the fractions by various isolated sediment microorganisms, as determined by epifluorescence count. All fractions except the asphaltenes supported the growth of at least two of the isolates, although fractionation of degraded bitumen revealed that the saturate, aromatic, and first polar fractions were preferentially degraded.