Empirical factors for converting 3H thymidine incorporation into bacterial production were determined from samples taken in a mesotrophic lake. Twenty five diluted water cultures were conducted at "in situ" temperature (4-22° C) from April 1987 to February 1989. The conversion factors varied through the year ; the average conversion factor was 6.29 × 109 cells (2-10.7) per nanomole of thymidine incorporated into cold TCA precipitate and 7.18 × 109 cells (3.47-10.7) nmol-1 , when corrected for the increase in cell biomass which occured during the incubations. The validity of these factors, higher than the theoretical conversion factors values, is discussed considering the diluted water culture conditions and the data analysis methods.