The microbial surface and flocculability were qualitatively characterized through the combination of the surface thermodynamic and the extended DLVO approaches, with Ralstonia eutropha, a polyhydroxybutyrate-producing bacterium, as an example. The negativity of the zeta potential of R. eutropha decreased from the initial -19.5 to -11 mV in its cultivation with the consumption of glucose. The total interfacial free energy (Delta G(adh)) was changed from -80 to 28.5 mJ m(-2) in its entire growth process. This suggests that the bacterial surface changed from hydrophobic into hydrophilic, resulting in an alteration of its surface characteristics and flocculability in its different growth phases. As a result, the stability ratio of suspensions increased with the increasing cultivation time, indicating that the cell particles became more repulsive with each other and led to a more stable suspension of R. eutropha in its cultivation. The obtained information in this work might be useful for better understanding the surface characteristics and the flocculability and even manipulating its flocculability in the microbial growth process.