Native vacancy defects are studied in as-grown GaAs materials by positron-lifetime measurements. Direct evidence of native monovacancy-type defects is found in as-grown n-type Te-, Sn-, and Si-doped GaAs. However, the same evidence is not found in as-grown semi-insulating In- or Cr-doped GaAs or semi-insulating undoped GaAs or p-type GaAs(Zn). It is shown that the positron trapping and annihilation in the native defects are strongly dependent on the position of the Fermi level in as-grown n-type GaAs. It is concluded that the configurations of the native monovacancy defects in GaAs (Te or Sn) change with the position of Fermi level. Two Fermi-level-controlled transitions are found: one is located at 0.035\ifmmode\pm\else\textpm\fi{}0.015 eV and the other at 0.10\ifmmode\pm\else\textpm\fi{}0.02 eV below the conduction band. It is proposed that the two transitions correspond to two charge-state transitions of the arsenic vacancy ${V}_{\mathrm{As}{}^{2\mathrm{\ensuremath{-}}}\ensuremath{\rightarrow}{V}_{\mathrm{As}{}^{\mathrm{\ensuremath{-}}}}}$ and ${V}_{\mathrm{As}{}^{\mathrm{\ensuremath{-}}}\ensuremath{\rightarrow}{V}_{\mathrm{As}{}^{0}}}$, respectively.