With data from eight Canadian and eight Danish lakes, we used bacterioplankton production ([3H]thymidine incorporation) in multiple regression analyses to determine to which variable it is best related. We studied two size classes: < 1 µm and 1–3 µm. For the large bacteria (1–3 µm), primary production was the only statistically significant variable. For the small bacteria (<1 µm) abundance came first, followed by chlorophyll. Small bacteria have a more uniform reproductive rate. It is suggested that phytoplankton exudates are better substrates for large than for small bacteria. Temperature is not useful for predicting bacterial production, nor are concentrations of total organic C or dissolved organic C. Bacterial abundance increase with trophic status seems to be the cause of the correlation between bacterial production and the eutrophication gradient. The turnover time of large cells was shorter than that of small cells in 30 of 37 cases. A large bacterium represents ∼10–100 times more biomass per cell, so large bacteria are often the main source of organic matter synthesis.