The defect spectra formed during plastic deformation of semiconductors have been investigated by means of positron lifetime spectroscopy. A peculiar feature of the average positron lifetime at low temperatures gives evidence of a new metastable deformation-related defect configuration in GaAs. Nonconservative dislocation motion such as jog dragging is an effective source of vacancies and vacancy clusters. The point defects may be agglomerated in the dislocation core. Vacancies, which are released as a result of dislocation cutting and jog dragging, form stable clusters. The stability, the size, and the related positron lifetime of such clusters have been investigated theoretically by density-functional calculations.