Interference between the first two modes of Earth-ionosphere waveguide propagation at the high end of the VLF band (<18 kHz) increases with distance from the transmitter out to very large distances and can add amplitude perturbations to the phase perturbations (trimpis) produced by lightning-induced electron precipitation (LEP) on the great circle path. Since the two modes have slightly different phase velocities, an interference pattern or standing wave is formed which is shifted slightly along the propagation path by the LEP-induced change in differential phase velocity. The modal effect at the receiver depends on the local gradient (along the great circle path) of amplitude with respect to the differential phase. Since this differential or mode beat phase varies with frequency, measurement of the resultant amplitude at two close frequencies enables an estimation of the modal effects. In this study, measurements were made at Dunedin at the two MSK frequencies, 22250 Hz and 22350 Hz, of the transmitter NWC, during a night of frequent ''one-dimensional'' trimpis (i.e., those produced by large-area LEP occurring close to the great circle path) and of strong and varying modal interference. Modal generation or modification of trimpi amplitude was related to the local gradient of amplitude as expected. From these results it was deduced that modal modification of ''echo trimpis'' (those produced by small area LEP occurring well off the great circle path). ¿ American Geophysical Union 1989