The ring-opening polymerization of epsilon-caprolactone initiated by ethyl magnesium bromide was studied. Because ring-opening polymerization is extremely fast in the monomer phase, we focused on solution polymerization in toluene. The solution polymerization of epsilon-caprolactone was characterized by the linear dependence of degree of conversion on polymerization time, which we described by a first-order kinetic equation. The molar mass, determined by SEC using a light scattering detector, was in agreement with that calculated from the degree of conversion attained and the ratio of epsilon-caprolactone to ethyl magnesium bromide. On this basis, we propose a mechanism for the effect of ethyl magnesium bromide on epsilon-caprolactone polymerization and provide evidence for this mechanism by isolating a novel stable complex of magnesium bromide with epsilon-caprolactone.