A stainless steel shielded hot-wire viscosity sensor for use in fermenters was developed. The sensor responded well to changes in viscosity in a Newtonian fluid and to changes in the apparent viscosity in a non-Newtonian fluid over a wide dynamic range. The sensor was able to recognize the viscosity at a very low shear rate, and to withstand high-pressure steam sterilization. The sensor was used to monitor xanthan production by Xanthomonas campestris and the cell growth of Nicotiana tabacum BY-2. The sensor responded well to changes in the apparent viscosity of both culture fluids. Because of its simple structure and wide dynamic range, the shielded hot-wire viscosity sensor is expected to have wide applicability as an in-line viscosity sensor for monitoring the metabolite concentration and cell density in fermentation processes.