The interaction parameter of moving dislocations with electrons and with phonons was determined in aluminum, in the temperature range 10 to 250°K. A new technique, involving a "dynamic bias stress", was developed for measuring ultrasonic-attenuation changes ∆α. The numerical values of the interaction parameter were obtained from the measured ∆α by means of an analysis which does not require any knowledge of the dislocation density or of other inaccurately known features of the dislocation network. The results indicate that the dislocation interaction with electrons is temperature independent and the interaction with phonons increases with increasing temperature. These results are consistent with theoretical predictions.