  We have used high-resolution neutron Larmor diffraction and capacitative
dilatometry to investigate spontaneous and forced magnetostriction in undoped,
antiferromagnetic YBa$_2$Cu$_3$O$_{6.0}$, the parent compound of a prominent
family of high-temperature superconductors. Upon cooling below the N\'eel
temperature, $T_N = 420$~K, Larmor diffraction reveals the formation of
magneto-structural domains of characteristic size $\sim 240$~nm. In the
antiferromagnetic state, dilatometry reveals a minute ($4 \times 10^{-6}$)
orthorhombic distortion of the crystal lattice in external magnetic fields. We
attribute these observations to exchange striction and spin-orbit coupling
induced magnetostriction, respectively, and show that they have an important
influence on the thermal and charge transport properties of undoped and lightly
doped cuprates.
