Counting bacteria in natural water samples by cultivation yields only low recovery efficiencies (ca. 1%), compared to total counts obtained after 4,6-diamidino-2-phenylindol (DAPI) staining. In order to optimize the cultivation of heterotrophic planktonic bacteria from Lake Constance (Germany), selected parameters of the medium composition were modified. The most important factor was the concentration of organic substrate (nutrient broth plus yeast extract), which significantly influenced the "most probable number" obtained in liquid growth medium. Reduced oxygen concentrations (3-12%) lowered the "most probable number". Addition of N-acyl homoserine lactones to the medium increased the cultivability slightly. Low substrate concentrations [0.03-0.06% (w/v)], an incubation atmosphere of 21% oxygen at 16 degrees C for 4 weeks were optimal and increased the cultivability ("most probable number" related to total bacterial counts) to an average cultivability of 18+/-11%, (n=8). The results indicate that cultivabilities of heterotrophic bacteria from lakewater samples can be significantly increased by modifying the cultivation methods.