We simultaneously assessed the metabolic activity and viability of individual bacterioplankton cells in the coastal and open North Sea. Three different techniques were applied to determine cell features related to the physiological status of the cell. SYBR Green I was used to estimate the nucleic acid content of the cell. Propidium iodide (PI) stains cells with a compromised cell membrane, commonly interpreted as indicative of dead cells. Microautoradiography (MA) with radiolabeled glucose and leucine was applied to indicate metabolically active cells. The relative abundance of metabolically active cells determined by MA was usually