The importance of matching the phase velocity of the incident plane wave to the numerical phase velocity imposed by the numerical dispersion of the three-dimensional (3-D) finite difference time domain (FDTD) grid is demonstrated. In separate-field formulation of the FDTD method, a plane wave may be introduced to the 3-D computational domain either by evaluating closed-form incident-field expressionss or by interpolating from a 1-D incident-field array (IFA), which is a 1-D FDTD grid simulating the propagation of the plane wave. The relative accuracies and efficiencies of these two excitation schemes are compared, and it has been shown that higher-order interpolation techniques can be used to improve the accuracy of the IFA scheme, which is already quite efficient. ¿ 1997 American Geophysical Union