Weak field piezoelectric coefficient and strain were investigated in ferroelectricSrBi2Ta2O9(SBT) thin films by means of optical interferometry. Though the maximum polarization and dielectric constant were small enough (7&mgr;C/cm2and 150, respectively), the piezoelectric coefficient attained 17 pm/V under the dc electric field. This value is comparable with the piezoelectric coefficients reported previously on poled SBT ceramics. Electrically induced strain of5⋅10−4was observed using bipolar driving field of 300 kV/cm. No piezoelectric fatigue was found until109switching pulses, in agreement with polarization data. The piezoelectric properties and strains were successfully described by using a simple electrostriction equation with the effective electrostriction coefficient of 0.1m4/C2.The electromechanical behavior of SBT films suggested no or weak contribution of non-180° domain walls to the strain response. ©1997 American Institute of Physics.