[en] Evidence for stable octupole deformation in neutron-rich nuclei bounded by Z=54-58 and N=85-92 is described. More vibrational type octupole modes are observed to either side of this region in ¹⁴⁰Ba and ^152,154Nd. The largest stable octupole deformation (β₃∼0.1) is reported in ¹⁴⁴Ba_gg. The theoretically predicted quenching (β₃→0) of stable octupole deformation with increased rotation is observed in ¹⁴⁶Ba. There is good agreement between theory and experiment for the strongly varying electric dipole moments as a function of mass for ^142-148Ba. In odd-A ¹⁴³Ba and odd-Z, ¹⁴⁵La, two pairs of positive and negative parity bands with opposite spins, parity doublets are observed. In ¹⁴⁵La the ground band with symmetric shape coexists with the asymmetric octupole shape which stabilizes above about spin 19/2⁺. The isotope ¹⁰⁹Mo was identified and a new region of stable octupole deformation is identified in ^107,109Mo centered around N=64-66 as earlier predicted. This is the first case of stable octupole deformation involving only one pair of orbitals. (c) 1999 American Institute of Physics