Slow cooling of a supersaturated solution Of L-glutamic acid, with continuous or pulsed agitation during cooling, is sufficient to stabilise the a-polymorph for crystallisation times of 24h at 45 degreesC. Rapid cooling with agitation or slow cooling without agitation favours formation of the stable beta-polymorph. A reduction in average particle size and crystal quality of the alpha-crystals was observed following agitation and there was a marked absence of beta-inclusions on the surface of and inside the alpha-crystals under these conditions. Two hypotheses are presented to explain the stabilisation of the alpha-polymorph namely, (i) that agitation is sufficient to disrupt nucleation of the beta-crystals on the surface of the alpha-crystals and (ii) that a-crystals formed with agitation during slow cooling are small and poorly formed and lack the necessary well-formed crystallographic facets on which the beta-form can nucleate. That the beta-polymorph is favoured during rapid cooling with agitation can be explained in terms of the reduced period of agitation. (C) 2004 Elsevier B.V. All rights reserved.