A number of reports have suggested that nanometric thin films of yttria-stabilized zirconia (YSZ) deposited on MgO can support high ionic conductivity, but the results remain controversial and difficult to repeat. In this work, sub-100-nm-thick YSZ films have been deposited on single-crystal MgO substrates with different crystallographic orientations and sourced from different companies. The growth of YSZ on MgO (100) was found to be unstable: both (111)-oriented films with polycrystalline structure and (100)-oriented films with cube-on-cube epitaxy were observed despite seemingly identical deposition conditions. On MgO (110) and MgO (111) substrates, the growth of YSZ was more stable with high degrees of texture in the (110) and (111) film directions, respectively. Ionic conductivities of the films were measured with impedance spectroscopy, and conductivity values were consistently near or slightly below that of a YSZ single crystal.