High-spin states in 123I have been populated in the 80Se(48Ca,p4n)123I reaction at 207 MeV and γ-ray coincidence events have been recorded with the Gammasphere spectrometer. The level scheme of 123I has been extended up to spin I=63/2. The nucleus undergoes a shape transition from moderately deformed states with collective rotation at low spins to noncollective oblate configurations at higher spins. Maximally aligned terminating states involving all nine particles outside the 114Sn core and states with one particle antialigned are identified. A large number of weak transitions feed the terminating states. Cranked Nilsson-Strutinsky calculations have been performed to determine possible configurations for the observed energy levels
