By means of a test particle model we have studied the time evolution of the velocity distribution function of newborn cometary ions in the presence of strong hydromagnetic turbulence. The test particle model employs a realistic spectrum of hydromagnetic turbulence corresponding to the observations of the International Cometary Explorer spacecraft at comet Giacobini-Zinner. It is found that pitch-angle scattering processes can rapidly result in the formation of a shell distribution which has been observed near comets Giacobini-Zinner and Halley. The model also indicates that radial diffusion in velocity space develops on a much longer time scale. ¿ American Geophysical Union 1987