The results of ab initio valence-bond calculations, with an STO-6G basis set, are reported for the ground state of ozone. Both Heitler–London and localized molecular-orbital procedures have been used to construct the wavefunctions for the σ-bonds. Hybridization of the orbitals on each atom has been determined by energy considerations. In accord with previous results, the primary Lewis structure is calculated to be the spin-paired diradical structure (6) with two σ-bonds, and a long, or formal, π-bond associated with the terminal atoms. However, the sum of weights for four Lewis structures (11)–(14), which arise from polarization of either σ-bond in (6), is calculated to exceed the weight for (6). Particular attention is given to the calculation of energies for “increased-valence” structures (1) and (2), which are constructed by spin-pairing the two unpaired electrons of ground-state O2 with those of a ground-state oxygen atom. If a two-centre molecular orbital is used to accommodate the two electrons of the O—O σ-bond in each of these structures, then resonance between these structures is equivalent to resonance between numerous Lewis structures including (6)+(11)–(14), when Heitler–London procedures are used to formulate the wavefunctions for all electron-pair bonds in the Lewis structures. The results of calculations which utilize this approach indicate that resonance between increased-valence structures provides the primary valence bond representation for the O3 ground state. Such structures are stabilized slightly by interaction with the Lewis structures that involve excited states for the O atom and the O2 molecule. The resulting energy for resonance between these four valence-bond structures lies at least 0.134 au below the STO-6G RHF-MO energy of –224.091 au.