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In order to obtain a superior thixotropic property even in a high strength 7075 aluminum alloy, a billet of an Al-5.5%Zn-2.5%Mg-1.5%Cu alloy with a nondendritic fine-grained structure has been tried to produce by combining a simple mechanical stirring with the vertical semi-continuous casting process. We succeeded to get a good quality billet on the basis of the trials that had been carried out under the various stirring conditions, especially focussing on the position of a stirring rotor, the shape of a rotor and the volume of a stirring chamber. When the temperature within a stirring chamber was nearly constant just below the liquidus temperature and dropped drastically near the liquid-solid interface during casting, a long sound billet with relatively smooth surface could be produced. And the billet had not only a uniformly fine-grained structure with the average grain size of ∼30 μ m, but also little macroscopic segregation of the solute elements along the radius at the various positions of the billet.