Semi‐insulating undoped GaAs was plastically deformed and then investigated by positron lifetime spectroscopy. Strains between 0% and 40% and temperatures of deformation of 450, 500, and 600 °C were investigated, with detailed investigations carried out for the lowest temperature of deformation. Between 0% and 4% strain a reduction of the grown‐in vacancy response takes place simultaneously with a slight increase in vacancy cluster size to 2 or 3 vacancies. Between 4% and 6% strain a very substantial increase in vacancy production occurs but nearly all of these vacancies are clustered into voids with a radius of about 50 A˚ and density of the order of 1013–1014cm−3. The total concentration of vacancies necessary to produce these voids is 1017–1018cm−3. This clearly shows that vacancies are formed upon deformation and that they are mobile at 450 °C. The small vacancy clusters (2 or 3 vacancies) are present at a concentration of about 5×1016cm−3, the same as for the 4% strained samples. Upon further deformation to 20% strain the overall defect concentration becomes so high that all positrons become trapped for which reason no absolute defect concentrations can be deduced. The dominant defect types can nevertheless be identified as voids (with average size of 20 A˚), two‐ or three‐vacancy clusters, and shallow traps. These shallow traps have a positron binding energy of about 40 meV. Isochronal and isothermal annealing of 40% strained samples shows that heat treatment reduces the void concentration but increases the average void size, reduces the small vacancy cluster concentration, but results only in a small decrease in shallow trap concentration. The shallow traps are likely the dislocation lines themselves and the small vacancy clusters appear to be associated with the dislocation lines.