Bacterial secondary production in lake water was measured by(3)H-thymidine incorporation into DNA. The application of the method to freshwater systems studied required (1) thymidine concentration > 10 nM (10-25 nM) evaluated from isotope dilution by varying the specific activity of labeled thymidine, (2) short incubation periods less than 1 hour, and (3) partial purification of the DNA fraction for measuring(3)H-thymidine incorporation. During 2 diel studies, bacterial productivity was compared to phytoplankton primary production and extracellular release of organic carbon. Diel changes in bacterial growth suggested substantial activity during the morning and evening. Possible mechanisms of control of bacterial growth, such as extracellular release of organic carbon, are discussed.