Electrostatic force is investigated as one of the components of the adhesion force between Bacillus thuringiensis (Bt) spores and planar surfaces. The surface potentials of a Bt spore and a mica surface are experimentally obtained using a combined atomic force microscopy (AFM)−scanning surface potential microscopy technique. On the basis of experimental information, the surface charge density of the spores is estimated at 0.03 μC/cm2 at 20% relative humidity and decreases with increasing humidity. The Coulombic force is introduced for the spore−mica system (both charged, nonconductive surfaces), and an electrostatic image force is introduced to the spore−gold system because gold is electrically conductive. The Coulombic force for spore−mica is repulsive because the components are similarly charged, while the image force for the spore−gold system is attractive. The magnitude of both forces decreases …