Excited states in 155 Tm have been populated via the reaction 144 Sm( 16 O, p4n) 155 Tm at a beam energy of 118 MeV. The ground-state band has been extended and a new side band of the ground-state band is identified. E-GOS curves and potential energy surface calculations are employed to discuss the structure evolution of the ground-state band. The newly observed side band in 155 Tm is discussed based on the spin/energy systematics. In particular, the phenomenon of seniority inversion is proposed in 155 Tm, and a systematic study of this phenomenon in the A ≈ 150 mass region is performed.