The polymerization of 1,3-cyclohexadiene with nickel bis(acetylacetonate) activated by methylaluminoxane affords poly(1,3-cyclohexadiene) in high yields; the same catalyst is unable to polymerize larger conjugated cyclic diolefins or copolymerize 1,3-cyclohexadiene with styrene. In the latter case, the homopolymer of the diolefin is obtained. The catalyst activity increases with increasing reaction temperature, nickel concentration, and aluminum/nickel ratio or with the addition of triisobutylaluminum to the reaction medium. The obtained poly(1,3-cyclohexadiene) samples are high-melting crystalline polymers (melting temperature similar to 320 degrees C) that are insoluble in all common organic solvents. With bis(cyclopentadienyl)nickel in place of nickel bis(acetylacetonate), the activity is much lower, but the polymer is more stereoregular, as indicated by the slightly higher value of the melting temperature.