We present correlated particle and magnetic field measurements by the ISEE 3 spacecraft for the loop structure behind the interplanetary traveling shock event of November 12, 1978. Following the passage of the turbulent shock region, we observe strong bidirectional streaming of low-energy protons for approximately 6 hours, corresponding to a loop thickness of about 0.07 AU. This region is also characterized by a low relative variance of the magnetic field, a depressed proton intensity, and a reduction in the magnetic power spectral density. Using quasi-linear theory applied to a slab model, we derive a value of 3 AU for the mean free path during the passage of the closed loop. We infer from this observation that the proton regime associated with the loop structure is experiencing scatter-free transport and that either the length of the loop is approximately 3 AU between the sun and the earth or else the protons are being reflected at both ends of a smaller loop.