Primary Si crystals are usually present in the cast microstructures of near-eutectic, eutectic, and hyper-eutectic Al-Si base alloys. Three-dimensional digital images of individual primary Si crystals present in a permanent mold cast unmodified Al-12 wt% Si-1 wt% Ni base alloy are reconstructed using a combination of montage serial sectioning and three-dimensional digital image processing techniques. Octahedral, prismatic, and plate-like three-dimensional morphologies of the primary Si crystals are present in the microstructure. Some of the primary Si crystals contain interior regions/islands of Al-alloy that are completely enclosed in the corresponding Si crystals indicating certain variations in the crystal growth velocities during the evolution of these crystals. The boundaries of these interior regions/islands are non-faceted smooth and curved indicating re-melting of the Al-rich islands and re-dissolution of some Si near these internal boundaries in the Al-alloy as a result of the heat generated by liquid-to-solid transformation of Si away from the islands. (C) 2009 Elsevier B.V. All rights reserved.