Abstract The difficulty in collecting fine particles (0.1 to 1 μm range) may require the use of fibrous filters with gas velocity of only a few centimeters per second. Kuwabara's cell model has been quite successful in the determination of the viscous flow field in aerosol filtration by fibrous filters where the Reynolds number is less than one. In this paper, Kuwabara's cell model is extended to the electrostatic filtration in a viscous flow field for both charged and dielectric spherical particles. The particles are assumed to be large enough to allow diffusion to be neglected. The utilized model takes into account the interference effects, namely the influence of neighbouring fibers on the flow field, in a more effective way than the single and three fiber models. A formula relating theoretical filter efficiency to a deposition coefficient is proposed. The filter efficiency obtained using this formula is compared to that given by the single fiber theory.