We have examined high time resolution magnetic field data for different separations of ISEE 1 and 2 in the Earth's ion foreshock region to study the coherence length of upstream ULF waves. By examining the correlation coefficients of the low frequency waves as a function of separation distance, we find that the correlation coefficient depends mainly on the separation distance of ISEE 1 and 2 transverse to the solar wind flow. It drops to about 0.5 when the transverse separation is about 1 RE, a distance much larger than the proton thermal gyroradius in the solar wind. Thus the coherence length of the low-frequency waves is about one Earth radius, which is of the order of the wavelength, and is consistent with that estimated from the bandwidth of the waves. ¿ American Geophysical Union 1990