Fast scanning chip calorimetry (FSC) was used to prepare isothermally crystallized samples of polyamide 66 (PA 66) in a wide range of temperatures between 70 and 230 degrees C for subsequent analysis of the semicrystalline morphology by X-ray diffraction, polarized-light optical microscopy and atomic force microscopy. At high crystallization temperatures stable triclinic alpha-crystals of lamellar shape, organized within a spherulitic superstructure are formed. At low crystallization temperatures, in contrast, the less stable pseudohexagonal gamma-crystal structure/mesophase develops. The mesophase of PA 66, which forms at temperatures close to the glass transition, is of grainy, non-lamellar habit, and not organized within spherulites. The formation of such qualitatively different semicrystalline morphologies of PA 66 is suggested to be caused by different densities of crystal nucleation, supported by observation of a bimodal temperature-dependence of the crystallization rate. The experimental findings reported in this work are important to allow tailoring of the microstructure of PA 66 by variation of the conditions of processing as well as contribute to the ongoing research about crystal nucleation in polymers.