Variations of rheological properties of culture broth during the production of biopolymer by an alkali tolerant Bacillus sp. were investigated. Correlations among the rheological characteristics of culture broth, cell growth and biopolymer production were examined. Rheology of the culture broth changed in the course of fermentation. The culture broth showed a non-Newtonian flow behavior, as the viscosity and pseudoplasticsity increased during the cultivation. The rheological parameters such as flow index, consistency index, yield stress and apparent viscosity during the cultivation were not influenced by the cell growth, but significantly related to the biopolymer synthesis. Changes in the rheological parameters of the broth were affected not only by the biopolymer concentration, but also by the progress of fermentation. Some rheological parameters showed maximum values just before the completion of biopolymer production and substrate consumption. Hence, it was shown that the rheological characteristics of the culture broth could be used as a good indicator for the detection of the progress or completion of fermentation.