Dynamic mobility spectra of suspensions of semiconducting pyrolusite particles (manganese dioxide) have been measured by means of electroacoustics in different supporting electrolytes. The magnitude of the complex dynamic mobility spectra of these particles is small. At low frequency the magnitudes increase with increasing electrolyte concentration, yet the high-frequency magnitudes are independent of ionic strength. At fixed electrolyte concentration the low-frequency phase lag of the dynamic mobility increases with frequency. At high frequency, however, the phase lag shows a significant reduction, which increases with increasing electrolyte concentration. The trends observed in the dynamic mobility spectra of pyrolusite are interpreted qualitatively with O'Brien's model for the dynamic mobility of semiconductors.