The modal expansion method by Ittipiboon et al. (1991) for aperture-coupled microstrip antennas is combined with the Lorenz reciprocity and Poynting theorems to calculate the admittance of slots in the walls of nonradiating dielectric (NRD) waveguides. Expansion coefficients are determined from the induced field in the slot, and the normalized impedance of the slot with zero wall thickness. The effect of wall thickness is then incorporated by treating the slot as a stub guide with a certain propagation constant and a characteristic impedance. The thick slot is thereby treated as a transmission line, and theoretical values of normalized conductance and susceptance are given for examples of slots in NRD waveguides. Slot admittance is given as a function of slot inclination, and reasonable agreement is found with corresponding experimental data.