The molecular beam epitaxial growth of zinc blende (ZB) MgS on GaAs, GaP, and InP substrates has been investigated by X-ray diffraction and RHEED, with MgS layer strain varying between 3.1% compressive strain (GaP) and 4.4% tensile strain (InP). ZB MgS could be grown on all three substrates. X-ray diffraction showed substantial MgS relaxation during growth before conversion to the rock salt phase. Results are compared with predictions that stable growth on GaP is unlikely and relaxed ZB MgS does not grow in layers over a few Å thick. Our results imply growth of ZB MgS is truly metastable.