Abstract: Aqueous mixtures of fermentation-produced polysaccharides and cellulose derivatives such as hydroxyethylcellulose are used as drilling fluids. They exhibit low viscosity at high shear rates and high viscosity at low shear, causing them to be ideally suited for rapid drilling rates and good cuttings carrying capacity. These fluids are stable in the presence of high calcium ion and salt concentrations and can be used in drilling troublesome shale sections without hydration problems. The addition of KCl in modest amounts improves shale stabilization.