Fe ultrathin films on GaAs(001) substrates were prepared by thermal deposition (TD) and pulse laser deposition (PLD) using MBE. Conversion electron Mössbauer spectroscopy (CEMS) was employed to investigate the atomic arrangement of Fe at the GaAs(001) interface. The magnetic anisotropies were studied by FMR. They have strong interface and bulk contributions which undergo several transitions with increasing film thickness. The most pronounced effect was observed in the in-plane interface uniaxial anisotropy K^int||,u. K^int||,u=0.10 ergs/cm² for the TD films thinner than 30ML. For thicker samples it decreased to K^int||,u=0.03 ergs/cm² which is equal to that for the PLD samples. It will be shown that these transitions in magnetic anisotropies are driven by B₁ and B₂ magneto-elastic energies.