The marine photosynthetic bacterium Rhodovulum sulfidophilum produces extracellular nucleic acids involved in its flocculation. Previously, we showed that the RNA fraction of these extracellular nucleic acids released into the culture medium contains mainly non-aminoacylated fully mature-sized tRNAs and fragments of 16S and 23S rRNAs. Here, we report the characterization of extracellular DNA itself and its production during cultivation. No differences were detected in nucleotide sequence between the intracellular DNA and extracellular soluble DNA on Southern blotting. Whole intracellular DNA seemed to be released from the cell. The bacterial floc was degraded by deoxyribonuclease or ribonuclease treatment, indicating that at least the extracellular DNA and RNAs in the floc are involved in the maintenance of the floc. When cultivated in nutritionally rich medium, the bacteria formed small flocs and produced large amounts of extracellular DNA, which were solubilized in the medium. In nutritionally poor medium, however, huge flocs of cells appeared and almost no extracellular soluble DNA was observed in the medium. As the floc was degraded by deoxyribonuclease treatment, it seems likely that the extracellular soluble DNA observed in the rich medium may be incorporated into the large floc and play a role in floc maintenance in poor medium. Addition of an inhibitor of quorum sensing, alpha-cyclodextrin, inhibited huge floc maintenance in the nutritionally poor medium. In the presence of alpha-cyclodextrin, the floc was rapidly degraded and extracellular soluble DNA production increased.