The defect formation during plastic deformation of GaAs has been investigated by means of the analysis of stress–strain curves including relaxation experiments and positron lifetime spectroscopy. The deformation creates a mixture of dislocations and point defects, which contribute to the positron lifetime spectra. Vacancy clusters can already be formed at rather low deformation temperatures via jog dragging. The concentration of generated defects depends on the number of activated slip systems for single-slip or multiple-slip orientation of the deformation axis.