3-Methyl-1,3-pentadiene (mixture of E and Z isomers) was polymerized with the systems COCl2(PRPh2)(2)-MAO (R = methyl, ethyl n-propyl, isopropyl, cyclohexyl), obtaining crystalline, essentially isotactic 1,2-poly(E-3-methyl-1,3-pentadiene)s, as indicated by IR and NMR (H-1 and C-13 in solution) analyses. The isotactic content was found to depend on the type of catalyst used (i.e., type of phosphine ligand bonded to the cobalt atom), increasing with decreasing the bulkiness of the phosphine ligand. The same cobalt systems were also used to polymerize 1,3-pentadiene and 1,3-hexadiene, obtaining polymers having the same 1,2 structure, but an opposite tacticity (syndiotactic instead of isotactic). The role played by the monomer structure in determining the polymerization stereoselectivity is discussed.