The galvanic initiation process is well known to allow direct electroless metal deposition on non-catalytic metals. In this paper it is shown that the galvanic initiation process is also suitable for the direct metallization of semiconductors without a previous activation step of the surface. A simple contact with a Ni sheet and the semiconductor surface, dipped in the Ni plating bath, leads to the initiation of Ni deposition on the semiconductor. The purpose of this paper is to describe our work on the galvanic initiation mechanism of electroless nickel deposition on a n-GaAs substrate. Early stages of electroless nickel plating are investigated using electrochemical measurements and morphological and structural observations of the deposits (SEM, Castaing X-probe and RHEED). With thermodynamical considerations, the galvanic initiation mechanism of metal deposition on n-GaAs is evolved.