This study investigates the relationships between biovolume and biomass of naturally derived marine bacterioplankton. The authors collected natural bacterioplankton, filtered them to remove larger particles, and diluted the cultures in unenriched seawater to grow cells comparable to those in natural seawater. They measured cell size using epifluorescence photomicrography and corrected for cells passing through glass-fiber GF/F filters. The average per-cell carbon biomass was found to be relatively constant at 20 ± 0.8 fg of C, while the biovolume-to-biomass conversion factor averaged 0.38 ± 0.05 g of C cm⁻³, which is about three times higher than previously estimated. The C:N ratio was 3.7 ± 0.2. The results suggest that natural marine bacterial biomass and production may be higher than previously thought, and that variations in bacterial size do not necessarily reflect variations in biomass per cell. The study also highlights the importance of accurate size measurement criteria and the need for a constant biovolume-to-biomass conversion factor for field data.This study investigates the relationships between biovolume and biomass of naturally derived marine bacterioplankton. The authors collected natural bacterioplankton, filtered them to remove larger particles, and diluted the cultures in unenriched seawater to grow cells comparable to those in natural seawater. They measured cell size using epifluorescence photomicrography and corrected for cells passing through glass-fiber GF/F filters. The average per-cell carbon biomass was found to be relatively constant at 20 ± 0.8 fg of C, while the biovolume-to-biomass conversion factor averaged 0.38 ± 0.05 g of C cm⁻³, which is about three times higher than previously estimated. The C:N ratio was 3.7 ± 0.2. The results suggest that natural marine bacterial biomass and production may be higher than previously thought, and that variations in bacterial size do not necessarily reflect variations in biomass per cell. The study also highlights the importance of accurate size measurement criteria and the need for a constant biovolume-to-biomass conversion factor for field data.