Ab initio 3-21G calculations have been performed for the [1,5]-H shift in cis-1,3-pentadiene. A transition state of C, symmetry has been compared with one of C, symmetry. The lowest energy configuration of this latter structure has Bl symmetry and must therefore be described by an open-shell calculation. The energy of this structure is favored by 5.2 kcal/mol over the one of C, symmetry. Both structures are found to be real transition states. Both the calculated reaction rates and the kinetic isotope effects are found to be considerably smaller than the observed ones. A mechanism is suggested in which tunneling takes place between high-vibrational states of the reactant and the product. It is shown that this mechanism is most likely for the transition state of Cb.symmetry. The calculated tunneling rates indicate that the [1,5]-H shift in cis-l,3-pentadiene mainly takes place via this mechanism.