The electronic structure of the self-trapped exciton (STE) in caesium iodide is investigated by a semi-empirical molecular-orbital method. The STE states are constructed by considering the relaxed electronic states of a cluster composed of an anionic diatomic I₂^- molecule surrounded by its nearest-neighbour Cs⁺ ions. For the electronic states, the 6s and 5d orbitals of the Cs⁺ ions are taken into account. Two types of STE states are obtained; the lower level manifold of each one is composed of a split triplet state and a singlet state. Both give rise to pi -emissions with a rather long radiative lifetime. The general expressions for the magnetic circular polarisation, the magnetic linear polarisation and the lifetime of the emissions are derived and discussed.