Abstract: The present invention relates to a process for preparing a Ziegler-Natta type catalyst based on vanadium and titanium. The process comprises (1) contacting within a liquid hydrocarbon a spheroidal support comprising (i) MgCl.sub.2 free from any Mg-C bond and (ii) an electron-donor (D1) free from labile hydrogen, successively with an electron-donor (D2) containing labile hydrogen and then with an organometallic compound capable of reducing vanadium and titanium compounds, (2) washing of the solid product with a liquid hydrocarbon and (3) then contacting the washed solid product with a vanadium compound and a titanium compound, both being soluble in the liquid hydrocarbon. The catalyst consists of spheroidal particles without fines, is very active in olefin polymerization and is particularly suitable for producing elastomeric copolymers of propylene.