We have estimated the magnetospheric local mass density using the frequency of the toroidal standing Alfv¿n waves detected with the magnetic (B) and electric (E) field experiments on the Combined Release and Radiation Effects Satellite (CRRES), which had a low-latitude elliptical orbit (6.3 RE apogee and 18¿ inclination). A 73-day period in 1991, which included several geomagnetic storms, was chosen for analysis because the spacecraft was located on the afternoon sector where toroidal waves are routinely excited. Dynamic spectra of the B and E fields were generated for each CRRES orbit for visual identification of toroidal mode ULF waves excited at a number of harmonics. The fundamental mode is the most persistent feature in the spectra, and this mode appears more clearly in the electric field than in the magnetic field. For statistical analysis we determined the fundamental frequency f1 from a separate set of electric field spectra that were computed in a moving 20-min time window with 10-min overlap. The frequency is in the range of 1.7--14.2 mHz. The local mass density consistent with f1 was obtained by numerically solving a magnetohydrodynamic wave equation that incorporated the Tsyganenko magnetic field model and an analytical form of field-line mass density variation that was derived from a recent study of toroidal harmonics. We combined the estimated mass density with the local electron number density ne derived from the plasma wave spectra at CRRES to obtain the local average ion mass M (=ρ/ne). A total of 1094 M samples were obtained, with the majority (95%) coming from the plasmatrough region (ne 100 cm-3).