Dislocation glide and climb during optical excitation of GaAs laser material have been studied. A dislocation which has been induced to glide under high excitation can be induced to climb by reducing the incident excitation. The climb proceeds in a distinctly different crystallographic direction to, and at a rate which is five or six orders of magnitude slower than, the glide. Furthermore, only the threading portion of the dislocation experiences climb, a misfit dislocation excited at the same external intensity for the same time undergoes no discernible growth. Transmission electron microscopy examination of the climb region shows that it consists of a highly convoluted giant dislocation dipole. The nature of this giant dipole has been determined by a calibrated technique and it has been shown to grow by vacancy climb. Small coherent precipitate particles have been identified for the first time in the neighborhood of the giant dipole.