The impact of different nucleation methods with controlled supersaturation on the polymorphism of L-glutamic acid crystals was studied in a 50 dm(3) semi-batch crystallizer, in which 1.5 M H2SO4 was fed into a 1.5 M monosodium glutamate solution. Supersaturation was controlled by a feedback loop based on measured ATR-FTIR spectra and on a thermodynamic model developed in house. Ultrasound-initiated nucleation produced pure a-polymorph (over 99.5 wt %) in the end product crystals at 1 mol/dm(3) supersaturation. Seeded crystallization also produced almost pure a-polymorph crystals (over 93 wt %), whereas spontaneous nucleation produced a polymorph mixture (from 30-70 wt %) under the same operating conditions. This indicates that supersaturation-controlled sonocrystallization can be used to control polymorphism in crystals. The novelty herein is the simplicity of the method directing the generation of a desired polymorph.