The theory is developed for the propagation of a semi-infinite plane crack in an aeolotropic material where the direction of propagation lies in a symmetry plane. The rate of work done by the applied forces in maintaining a constant velocity is calculated. This work is dissipated in the surface energy of the extending crack. This gives a relation between the applied force, the surface energy and the velocity of propagation as a function of direction. Such a relationship could afford insight into preferred directions of cracking in aeolotropic materials.