The thermally induced rearrangement of 7-(trans-butenylidene)bicyclo[4.1.0]heptane gives 8-methylbicyclo[5.3.0]undeca-1(2),9-diene with stereospecificity that is opposite that predicted by conservation of orbital symmetry. Further, when optically active starting material is thermolyzed, the product is racemic, but recovered starting material is unchanged. Identical results were obtained upon thermolysis of 6-(trans-butenylidene)bicyclo[3.1.0]hexane and of 2-oxo-7-(trans-butenylidene)bicyclo[4.1.0]heptane. These results require that the reaction proceeds through one or more achiral species, that at least one of them is planar, that they never return to starting material, and that there is a stereospecific pathway from the intermediate(s) to product, 6-Electron-6-orbital CASSCF calculations with a 6-31G* basis set reveal that the lowest energy singlet intermediates on the parent C6H8 energy surface are the achiral cisoid and transoid orthogonal 2-(1’-allyl)allyl biradicals. The cisoid form undergoes conrotatory closure to the methylenecyclopentene product via a transition state that is 2.8 kcal/mol lower in energy than the disrotatory transition state. The conrotatory transition state is consistent with the observed stereospecificity. The highest energy transition state is that from starting allylidenecyclopropane to an orthogonal 2-methylene-transoid-1,3-pentadienyl singlet species which then forms the transoid orthogonal bisallyl singlet biradical which undergoes bond rotation via a planar transoid vinyltrimethylenemethane transition state as well as slower closure to 2-vinylmethylenecyclopropane. Reopening of the latter material to a cisoid orthogonal bisallyl singlet biradical then provides the pathway for the stereospecific conrotatory closure to 3-methylenecyclopentene. Besides the cyclopropanes and methylenecyclopentene, the lowest energy species on the entire energy surface is the planar transoid vinyltrimethylenemethane triplet. The orthogonal bisallyl singlets are 8 kcal/mol higher in energy than the planar triplet at this level of theory.