The survival of Escherichia coli in bottom sediment (Lake Onalaska, navigation pool no. 7, Mississippi River) was studied by using in situ dialysis culture of sterile (autoclaved) and unsterile sediment samples. Bags made from dialysis tubing were filled with either course sand sediment (28.8% fine) or organic, silty clay sediment (77.2% fine) and placed at the sediment-water interface. Bags representing sterile controls, unsterile uninoculated controls, autoclaved inoculated sediment, and unsterile inoculated sediment were studied during a 5-day period for each sediment type. Daily most-probable-number determinations indicated that E. coli populations in unsterile inoculated sediment fluctuated between 5.3 X 10(2) and 2.2 X 10(3) bacteria per g of silty clay and between 3.0 X 10(3) and 1.4 X 10(4) bacteria per g of sand. Autoclaved silty clay sediment inoculated with 1.0 X 10(6) bacteria per g increased to 2.2 X 10(8) bacteria per g in 3 days. During the same period, autoclaved sand sediment inoculated with 1.2 X 10(5) cells per g increased to 5.4 X 10(7) bacteria per g. By day 5, populations in both cultures had decreased by 1 log. The ability of E. coli to survive for several days in aquatic sediment in situ suggests that fecal coliforms in water may not always indicate recent fecal contamination of that water but rather resuspension of viable sediment-bound bacteria.