We present an elaborate electron-spin-resonance study of the low-energy dynamics and magnetization in the ordered phase of the magnetically frustrated spinel ZnCr₂O₄. We observed several resonance modes corresponding to different structural domains and found that the number of domains can be easily reduced by field cooling the sample through the transition point. To describe the observed antiferromagnetic resonance spectra, it is necessary to take into account an orthorhombic lattice distortion in addition to the earlier reported tetragonal distortion which both appear at the antiferromagnetic phase transition.