Precipitation of the polymorphic compound L-glutamic acid was initiated by premixing aqueous solutions of sodium L-glutamate and sulfuric acid. Samples of the supersaturated solution were either subjected to vigorous post-stirring or left under quiescent conditions. For low supersaturation (S <= 13) without post-stirring aggregated platelet-shaped crystals of the stable beta polymorph formed while post-stirring generated large prismatic crystals of the metastable alpha polymorph. For high supersaturation (S >= 17) first smooth spheres were observed which transformed into rough spherulitic crystals of the beta phase. For high supersaturation it is proposed that the spheres are a metastable phase consisting of droplets formed by liquid-liquid phase separation. Subsequently from these spheres crystals of the stable beta phase nucleate. For low supersaturation without post-stirring aggregated beta platelets form according to the same mechanism, while with post-stirring concentration fluctuations are equalized and metastable alpha crystals nucleate from the homogenized solution. (c) 2006 American Institute of Chemical Engineers.