Ab initio calculations have been performed on the two modes of electrocyclic ring opening of Dewar benzene (1), as well as related structures. Both CASSCF and MP2 optimizations predict the existence of Mobius benzene (3, cis,cis,trans-1,3,5-cyclohexatriene) in a very shallow minimum ca. 100 kcal/mol above benzene (2). This structure has alternating single and double bonds, with one fully trans double bond. The barrier for pi bond rotation (3-->2) is predicted to be less than 3 kcal/mol. Stationary points have been located at the TCSCF/6-31G* level for both conrotatory and disrotatory ring opening of 1. The disrotatory path proceeds through a second-order saddle point, which appears to be a general characteristic of symmetry-forbidden reactions. At every level of calculation, conrotatory ring opening of 1 proceeds through a barrier that is 1.5-5.1 kcal/mol lower than disrotation. The predicted conrotatory enthalpic barrier of 24-28 kcal/mol agrees well with the experimental value of 25.1+/-2. Intrinsic reaction cordinate calculations suggest that conrotation from 1 may lead to either 2 or 3. Calculations also support the existence of trans-Dewar benzene (4), a substance originally suggested by Woodward and Hoffmann in 1971. This is predicted to be 158 kcal/mol above benzene, but still has a barrier of ca. 13 kcal/mol for ring opening to benzene.