Abstracts An exhaustive in-depth review of the biochemical literature concerning the biosynthesis of polyisoprenes, including natural rubber (NR, cis -1,4-polyisoprene c PIP) by rubber-producing plants, and the polymer chemical literature on biomimetic and related syntheses, leads us to postulate that the biosynthesis of polyisoprenoids in general and that of NR in particular, may proceed by a living carbocationic polymerization process. Our analysis led to the formulation of a “Natural Living Carbocationic Polymerization” (NLCP) mechanism in terms of accepted polymer chemical formalism, i.e., initiation, propagation, and equilibria between active and dormant species. A thorough analysis of the intermediates known to be involved in the biosynthesis of NR is consistent with our postulate.