The piezoelectric properties of SrBi4Ti4O15 and Bi4Ti2.95Nb0.04O12 have been compared as a function of applied ac stress. Evidence of domain-wall motion was observed in Bi4Ti2.95Nb0.04O12 but not in SrBi4Ti4O15. Examination of SrBi4Ti4O15 using transmission electron microscopy revealed that the material had undergone an orthorhombic-orthorhombic, paraelectric-ferroelectric, phase transition in which no piezoelectrically active, non-180 degrees, domain walls could be created. However, for the orthorhombic-monoclinic transformation in Bi4Ti2.95Nb0.04O12 new piezoelectrically active domain walls are allowed and were considered to move under applied stress. (C) 1996 American Institute of Physics.