Reducing sediments of Lake Mendota were challenged to demonstrate evidence of anaerobic conversion of l‐14C‐hexadecane to14CO2. Low levels of14CO2were detected above formaldehyde controls in anoxic sediment and bottom waters and in surface water sparged with nitrogen. Aerobic incubation of the same sediment and water demonstrated extensive conversion of l‐14C‐hexade‐cane to14CO2. The low levels of14CO2detected in anaerobic incubation were not increased by addition of nitrate or in a pre‐reduced medium containing nitrate and sulfate. An isotope‐recovery experiment demonstrated that 13.7% of added l‐14C‐hexadecane was converted to14C‐cell carbon and14CO2after 375 h incubation.