A study of the cosmic ray intensity data from high-latitude stations reveals an isotropic variation of ~30-day periodicity and peak-to-peak amplitude of 10% during the two months August--September 1979. Such an unusual modulation of cosmic ray intensity has not been observed before. It is clear that the variation is worldwide and in view of no significant north-south asymmetry, we have to visualize the modulation region responsible for the effect to be extremely large. The rigidity spectra of the variation is found to be similar to that of Forbush decreases. The results are discussed in terms of solar activity. No obvious physical mechanism is readily seen, to account for this intensity variation.