Optical observations of the CRRES plasma injection experiments revealed a highly structured ion cloud. In an attempt to understand these structures, we have developed a three-dimensional hybrid code to study the interaction between an injected plasma cloud and an ambient magnetized plasma. The primary advantage of this code is a seamless interface between a small plasma cloud, treated with fully kinetic particles, and an MHD fluid (ambient plasma). The results of the simulation are in good agreement with the observations. According to the simulation, a very dense ion core was formed as the neutral barium photoionized. The core polarized and moved in the initial injection direction via E¿B drift. The simulation also provided insight into a structured secondary neutral cloud created by Ba-Ba+ charge exchange. Specifically, the ion core split into two parts along the magnetic field, creating a nonuniform distribution of charge-exchanged neutrals. This structured ion core also leads to a herringbone structure in the trailing ion cloud. The code can be applied to similar problems including comets, coronal mass ejections, the Io-Jupiter interaction, and other plasma injection experiments. ¿ 2001 American Geophysical Union