We determine the band mobility of photoexcited electrons in cubic n-type Bi 12SiO 20. We measure a room-temperature mobility of 3.4 ± 0.5 cm 2/(Vs) that decreases monotonically to 1.7 ± 0.3 cm 2/(Vs) as the temperature is increased to 200 °C. We show that electrons in Bi 12SiO 20 form large polarons. Our results are predicted by strong coupling polaron theory if the band mass of the electrons is chosen to be 2.0 ± 0.1 electron masses. We determine the electron-phonon coupling constant and effective longitudinal optical phonon frequency required for this prediction from the available infrared reflectivity spectrum of Bi 12SiO 20.