Templating against two-dimensional (2D) regular arrays of square pyramidal pits etched in Si(100) wafers has been exploited to fabricate colloidal crystals with their (100) planes oriented parallel to the substrates (see Figure for an SEM image). The capability and feasibility of this method have been demonstrated by crystallizing 1.0, 0.48, and 0.25 mum polystyrene beads into 3D opaline lattices having such an orientation over areas as large as several square centimeters. Like their (111)-oriented cousins, these long-range ordered lattices of spherical colloids are useful in many areas such as photonics and porous materials. In particular the. ability to generate large colloidal crystals with adjustable spatial orientations will allow one to systematically inves- tigate their photonic band structures in an effort to elucidate the structure-property relationship.