EPR spectra of Fe-VO pairs were used to study how the order parameter of SrTiO3 varies with uniaxial stress applied to a (111) face. A second-order cubic-trigonal phase boundary appears above the stress-free transition temperature Ta. A first-order tetra-gonal-trigonal phase boundary is found below Ta. An independently determined Landau potential describes the results. © 1970 The American Physical Society.