Mercury allows one to test the concepts of magnetospheric dynamics in a very particular region of the parameter space governing planetary magnetospheres. One of the concepts to be tested is the field line resonance, which has become a paradigm in understanding the features of ULF pulsations in the Earth magnetosphere. Unlike field line resonances in the terrestrial magnetosphere, which are entirely described in terms of magnetohydrodynamics, the high frequencies of magnetospheric eigenoscillations at Mercury require a multicomponent treatment for the plasma. The model we present for field line resonances in the Hermean magnetosphere takes into account the plasma's multi-ion character. We find that the crossover frequency, i.e., a pure multi-ion effect, is a preferred frequency for field line resonances at Mercury. This allows us to infer the local plasma composition from observations of resonant pulsations. On the basis of a ULF pulsation event recorded by Mariner 10 during one of its Mercury encounters, we thus estimate the concentration of sodium ions in Mercury's magnetospheric plasma. ¿ 1999 American Geophysical Union