During Galileo's fifth close encounter with Jupiter's moon Ganymede (May 20, 2000), energetic particle observations revealed the existence of trapped ions within Ganymede's closed field line region. Previously only trapped electrons had been observed within Ganymede's magnetosphere. Furthermore, the measured electron distributions displayed a normal sinn(&agr;) pitch angle distribution as opposed to the butterfly distributions observed during the G8 encounter. The existence of trapped energetic ions in Ganymede's magnetosphere along with the existence of more stably trapped electrons shows the robustness of that magnetosphere and emphasizes the importance of modeling studies to determine particle drift paths within it and to reveal the processes that populate the closed field-line region. Qualitatively such drift paths seem able to unify details of the pitch angle distributions measured during the two encounters for which trapped particles have been observed. ¿ 2001 American Geophysical Union