The total energy of the symmetrical, bent O3 molecule is studied as a function of internuclear angle and internuclear distance. The method used is an ab initio molecular orbital calculation with a minimum basis set of Slater-type orbitals, plus limited configuration interaction. Best single configuration energies show the existence of two stable minima: a cyclic (60°) structure and a bent (115°) structure. The cyclic structure is preferred by 48 kcal/mol, contrary to experiment (116° 48′) Configuration interaction results with two low lying orbitals show the cyclic form still preferred by 6 kcal/mol. A transition state for the bent-cyclic pathway lies 26 kcal/mol (or less) above the bent form. Based on these results, a possible low energy path via the cyclic form is proposed for the thermal decomposition of ozone. It is also proposed that cyclic ozone may have been observed in the pulse radiolysis of oxygen.