We study the third-order electrical susceptibility of a fluid of nonpolar spherical molecules with linear polarizability and cubic hyperpolarizability. In the electrostatic dipole approximation, we derive an expression for the third-order susceptibility, which allows its evaluation from a property of the linear system. To second order in the linear polarizability, the correction to a mean field approximation, obtained by Buckingham, may be related to the correction to the Clausius-Mossotti formula for the linear dielectric constant. We present numerical results, obtained by computer simulation, for a system of hard spheres.