We show that bacterial viability can be calculated from the number of species arising in cultures comprised of seawater diluted to near extinction of the propagating organisms. Natural conditions are simulated by partitioning the culture against illuminated raw seawater to maintain a source of nutrients and a sink for waste products, and predators are eliminated by dilution to a statistical absence. Mean values for viability can be corrected for contaminants that may enter the culture by computing their probable entrance frequency as well. Viability determinations were found to be mildly sensitive to the number of species estimated to be present in the original sample. This novel procedure provides a significant improvement in the cultivation success of typical marine bacteria, and the formulations presented here lay the foundation for the quantitative analysis of the population structure.