Single‐crystal thin films of gallium‐substituted mixed garnets (Gd,Tm,Y)3(Fe,Ga)5O12, grown by liquid phase epitaxy, have been used as drive layer materials in contiguous disk bubble devices; a critical parameter for the selection of a specific composition is the value of the magnetocrystalline anisotropy constantK1. We have measured the values ofK1in a series of epitaxial thin films grown on (100) and (111) oriented gadolinium gallium garnet substrates.K1in the (100) oriented films were measured both by ferrimagnetic resonance and with an inductive hysteresigraph, whereas only resonance measurements were employed in the (111) oriented films. The values ofK1for each composition, obtained by different methods from films with different orientations, agree reasonably well with each other and are consistent with what would be expected from linear extrapolation of theK1’s of pure rare‐earth iron garnets after an empirical correction for gallium substitution.