Wide separations up to more than 1 RE between ISEE 1 and 2 during the second half of 1978 have been used to measure the correlation length of magnetic pulsaions in quasiparallel shocks. When the two spacecraft were less then a few hundred km apart, magnetic oscillations measured by magnetometers on both spacecraft exhibited virtually identical waveforms, but at distances of several thousand km, the two time series of field variation showed no detailed similarity at all. The correlation coefficients of the pulsations dropped from close to 1.0 spacecraft separations of less than 100 km to 0.2 for separations of greater than 800 km. A correlation length of several hundred km may be related to the gyroradius of return protons with energy typical of the peaks of diffuse and beam ion distributions.