The nucleation and growth mechanisms of the B polymorph of L-histidine (L-his) in a water-ethanol system were determined based on induction time measurements. The induction times were experimentally determined at different supersaturations and correlated using the models of mononuclear and polynuclear mechanisms. The primary nucleation mechanism of the B polymorph of L-his was identified as a polynuclear mechanism and the growth mechanism was found to be a two-dimensional nucleation-mediated growth. These mechanisms of the B polymorph of L-his will be used to characterize the polymorphic transformations and the overall crystallization rate of L-his.