Ultrasonic attenuation spectroscopy was successfully applied for in-situ determination of particle size during the crystallization of a molecular crystal in two different polymorphic forms. The crystallization of L-glutamic acid was carried out under different cooling conditions by which the a- or ß-polymorphic form of L-glutamic was obtained preferentially. It is shown that each polymorph yields a characteristic acoustic attenuation due to the different habits of the two polymorphs. While it was found that ultrasonic attenuation measurements were less sensitive than turbidometric measurements for the determination of the onset of crystallization, ultrasonic attenuation measurements proved to be well behaved and provided particle sizing data for the prismatic crystals of the a-form of L-glutamic acid and an estimation of the primary dimensions of the ß-form needles. Growth rate and supersaturation data were derived from particle size and solid concentration results and correlated.