Ethylene-styrene copolymers have been prepared in the presence of the catalytic system based on [(methyl)(phenyl)methylene(1-cyclopentadienyl)(9-fluorenyl)]zirconium dichloride, methyl- alumoxane, and Al((CH3)-C-13)(3). End-group analysis shows that in the initiation step the regiospecificity of styrene insertion into the Zr-(CH3)-C-13 bonds is prevailingly secondary.