Epifluorescent microscopy showed as many as 4 × 106 bacteria/mL in the turbid waters of the Athabasca River near the tar sand deposits in northeastern Alberta. The numbers were usually similar upstream and downstream (60 km) from pilot-mining operations. The majority of bacteria existed as free-living cells in spite of the fact there were high concentrations of suspended sediment present (average 220 mg/L) during the ice-free period. Fluctuations in bacterial concentration were positively correlated (r = 0.86, P < 0.05) with total organic carbon concentrations in the river water.