Sealed samples of specific aqueous hot-water-process-tar-sands-extraction streams and tailings pond sludges have been examined microbiologically and shown to contain compatible micro-organisms. Portions of these have been treated aerobically and anaerobically, with and without added nutrient, to encourage hydrocarbon utilization, and the results compared with experiments conducted with untreated and boiled control samples. While both aerobes and anaerobes appear to be indigenous in the matrices examined, aerobic treatment with nutrient present was found to give optimum solids-settling performance, and anaerobic treatment negligible short-term settling effect, although a slight improvement was noted for longer-term tests.