Thin films of MgO have been grown on Si(100) and GaAs(100) by ablation of a Mg target in an oxygen atmosphere using the fundamental frequency of a Nd:YAG laser. An ambient oxygen pressure of 5 x 10(-5) Ton : was found to be optimal for growth on both substrates; however, different substrate temperatures were required : 500 degrees C for growth on Si; 350 degrees C for growth on GaAs. This low growth temperature ensures minimal arsenic desorption from the GaAs substrate. The MgO films on both substrates are (100) oriented, as shown by X-ray theta-2 theta scans. The epitaxy of these crystals is confirmed by pole figure measurements of off-axis MgO reflections, which also indicate that the MgO lattices are oriented cube on cube with the substrate lattice, i.e. MgO (100) \\ Si (100) and MgO (100) \\ GaAs (100).