  We present an elaborate electron-spin resonance study of the low-energy
dynamics and magnetization in the ordered phase of the magnetically frustrated
spinel ZnCr$_2$O$_4$. We observe 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.
