At Skibotn in northern Norway, measurements have been made of the effect of powerful HF waves, from the heating facility at Ramfjordmoen, on the propagation of VLF waves from the Omega transmitter (12.1 kHz) near Aldra and on the 16.4-kHz transmissions (JXN) at Helgeland. The HF beam was deflectable in a north-south direction which enabled it to intersect the VLF propagation path. The VLF amplitude and phase perturbations were found to oscillate about zero as the beam was scanned across the propagation path, and typical daytime amplitude and phase perturbations had maximum magnitudes of ~0.1 dB and 0.5¿. The maximum values appeared when the heated beam was symmetrically located across the VLF path. A study of the temporal variability of the amplitude and phase perturbations at a fixed-beam deflection angle (38¿S) showed that large enhancements of up to two orders of magnitude occurred in the amplitude and phase perturbations at times when the total received VLF field strength passed through a minimum. Maximum amplitude and phase perturbations of 6 dB and 50¿ were recorded during one such minimum in the total received signal. Theoretical computations are presented which evaluate the diffracting effect of a heated patch of auroral ionosphere on multimode propagation in the earth-ionosphere waveguide. Computations of propagation under a series of ionospheres ranging from typical daytime to nighttime have been made, and the normal and enhanced amplitude and phase perturbations have been successfully modeled. Attempts at reproducing the perturbing effects of a mobile heated patch have not been nearly so successful, especially at large deflection angles.