Predictions of the amplitudes of the diurnal variation of cosmic ray intensity, based on calculations of the power spectra of interplanetary magnetic field fluctuations, are compared with measurements of the magnitude of the diurnal variations from Sulphur Mountain neutron monitor data for five solar rotations during mid-December 1965--April 1966, a period during which the interplanetary magnetic field data were also available. The nature of the general agreement between the derived theoretical predictions and the average observed variations over this limited period is pointed out.