When the ICE spacecraft flew past the comet Giacobini-Zinner on September 11, 1985, the ultra low-energy charge analyzer and energetic particle anisotropy spectrometer sensors observed an intensity gradient in the flux of fast (800--1400 km s-1) water group ions in the so-called transition region downstream of the cometary bow wave. We have combined the data from the two sensors and applied a single, self-consistent analysis technique to derive a single-particle spectrum from ~200--1600 km s-1. Using this information, along with the deduced bulk flow speed of the ions, we calculate a parallel diffusion coefficient in the transition region downstream of the cometary bow wave of (2.3¿0.5)¿1017 cm2 s-1; the corresponding scattering mean free path &lgr; is (6¿1)¿104 km. We also find that the parallel diffusion coefficient depends on the collision frequency &ngr;+ (&ngr;-) of water group ions with Alfv¿n waves, which we assume are propagating parallel (antiparallel) to the magnetic field. Our observations are consistent with &ngr;+=(1.6¿0.4)¿10-2 s-1 and &ngr;-/&ngr;+=0.3¿0.2; i.e., most of the waves are propagating in the antisunward direction. ¿ American Geophysical Union 1993