Experiments were conducted to determine whether production of heterotrophic bacterioplankton in a small meso-eutrophic lake was influenced by the dissolved inorganic phosphorus (DIP) supply. DIP may indirectly limit bacterial production by limiting phytoplankton, which in turn may limit the carbon available to bacteria. Direct DIP limitation of bacteria occurs where the availability of DIP for bacteria is insufficient to maintain growth. This work examined direct DIP limitation of bacteria by removing phytoplankton and incubating flasks with or without added P in the dark. Bacterial production was measured via the rate of incorporation of [H]thymidine ([H]TdR) into DNA. Bacterial abundance was followed with epifluorescent direct counts. Rates of [H]TdR incorporation were significantly greater in flasks with added DIP, and changes in cell abundances generally paralleled increases in [H]TdR incorporation. Even very small additions of P (0.05 muM) were sufficient to stimulate production. DIP addition to whole lakewater also stimulated [H]TdR incorporation relative to that in zero-addition controls, but there was not a concurrent increase in bacterial cell numbers. The stimulation of [H]TdR incorporation after DIP addition to whole lakewater was significantly less than the stimulation due to DIP addition to 1-mum-pore-size-filtered lakewater. In this study, addition of DIP caused as much as an eightfold stimulation of [H]TdR incorporation.