Weak field piezoelectric coefficient and strain were investigated in ferroelectric SrBi2Ta2O9 (SET) thin films by means of optical interferometry. Though the maximum polarization and dielectric constant were small enough (7 mu C/cm(2) and 150, respectively), the piezoelectric coefficient attained 17 pm/V under the de electric field. This value is comparable with the piezoelectric coefficients reported previously on poled SET ceramics, Electrically induced strain of 5.10(-4) was observed using bipolar driving field of 300 kV/cm. No piezoelectric fatigue was found until 10(9) switching 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 0.1 m(4)/C-2. The electromechanical behavior of SET films suggested no or weak contribution of non-180 degrees domain walls to the strain response. (C) 1997 American Institute of Physics.