A living anionic alternating copolymerization of ethylphenylketene (EPK) with 4-methoxybenzaldehyde (MBA) was achieved. When n-butyllithium was added to a mixture of EPK and MBA in tetrahydrofuran at -40 degreesC in the presence of an excess amount of lithium chloride, the copolymerization of these monomers proceeded via complete 1:1 alternating manner to afford the polymer with a narrow molecular weight distribution. A linear relationship was observed between the molecular weight and the monomer/initiator ratio, keeping a narrow molecular weight distribution. The structure of the obtained polymer was determined to be a polyester by IR spectroscopy together with the reductive degradation of the polymer by lithium aluminum hydride, which quantitatively afforded the corresponding diol to the repeating unit of the expected polyester structure. Both conversions of EPK and MBA agreed to a first-order kinetic equation with linear evolution between the molecular weight and conversion. These observations along with the successful results in two-stage polymerization indicate that the present copolymerization proceeded through a living mechanism.