The anionic polymerization of (R)-beta-butyrolactone initiated with either 18-crown-6 complexes of a potassium alkoxide or a simple carboxylate (reference initiator) proceeded with inversion of configuration. As a result (R)-beta-butyrolactone formed an isotactic poly((S)-beta-hydroxybutyrate) with these initiators at room temperature. Polymerization of (R,S)-beta-butyrolactone under the same conditions gave an atactic polymer, but at lower temperatures the predominantly syndiotactic form of poly((R,S)-beta-hydroxybutyrate) was produced from the racemic monomer.