A large cosmic ray intensity wave in which a single oscillation of the galactic flux with a peak-to-peak amplitude of about 4% and a period of approximately 27 hours, occurred on October 27--28, 1977. This unusual intensity profile was not associated with a classical Forbush decrease. Harmonic analysis in Universal Time of data from a global distribution of stations revealed that in each case the diurnal vector on October 27 differs significantly from those on other days before and after the event. Furthermore, the amplitude of the wave follows the relationship R1(i)=Ae&agr;&mgr;(i) where A and &agr;(=1) are constants and &mgr;(i) in the cosine of the angle between the direction of anisotropy and the asymptotic direction of viewing. The mechanism which produced this unusual oscillation is not yet understood.