The resistive (Coulter) method of counting and sizing particles in a conducting fluid has been extended to polystyrene spheres 900 Å in diameter, with a present detection limit near 600 Å, through the use of individual submicron pores etched in irradiated plastic sheet. The use of a nonionic surfactant and ultrasonic cleaning effectively relieves the problem of plugging. The particles may be driven through the pore by the electric field, without the use of pressure, to yield the vector sum of the electrophoretic and electro‐osmotic velocities. A new theory, yielding an upper limit to the resistive pulse on passage of a sphere, agrees well with data for spheres with diametersd<0.4D, whereDis the pore diameter, and complements a previous theory that gives a lower limit, valid ford>0.9D. We estimate that a detection limit near 250 Å will be attainable with the further development of current techniques.