Billabongs, lentic waterbodies common to the floodplain of Australian rivers, differ considerably from the lotic riverine environment in terms of hydrology, physiochemical characteristics, and biological assemblages present. As little is known regarding the bacterial ecology of billabong habitats, a comparison was made of the bacterial assemblages in the water column of seven paired river/billabong sites in the Murray-Darling Basin of southeastern Australia. Billabongs supported larger populations of bacteria (1-157 x 10(9) cells liter-1; 11-10,270-mu-g C liter-1) than did rivers (1-10 x 10(9) cells liter-1; 6-143-mu-g C liter-1). Phospholipid analyses confirmed that billabongs (14-111-mu-g phospholipid fatty acid liter-1) had larger bacterial populations than rivers (< 12-mu-g liter-1). Bacterial production, measured with H-3-leucine, was also greater in billabongs (0.28-3.05-mu-g C liter-1 hour-1) than rivers (0.05-0.62-mu-g C liter-1 hour-1). Production calculated from the frequency of dividing cells confirmed this conclusion, and suggested bacterial production in some billabongs could exceed 100-mu-g C liter-1 hour-1. An INT-formazan method indicated that usually < 25% of bacterial cells were active in either habitat, but this was probably an underestimate of the bona fide value. Turnover times of glucose were usually shorter in billabongs, and the cell-specific activity greater for billabong than river assemblages. The factors most likely to be responsible for the differences between the bacterial assemblages in rivers and billabongs relate to hydrological regime and the availability of organic carbon substrates.