The deformation of an isolated drop in an immiscible liquid undergoing oscillatory shear flow is experimentally investigated as a function of frequency and up to moderate amplitudes. Oscillatory shear flow is generated by using a parallel plate apparatus. Drop shape is observed by video light microscopy along the vorticity direction of the shear flow. The two principal axes and the orientation of the drop in the plane of shear are measured by image analysis. In the small amplitude range, the time dependence of the axes is also harmonic, but not in phase with the applied strain, the phase difference being a decreasing function of the imposed frequency. The linear range (where the major axis is proportional to the amplitude) extends up to strains of 0.5. Good quantitative agreement was found with the Palierne linear viscoelastic model (Palierne, J.F., Linear rheology of viscoelastic emulsions with interfacial tension, Rheol. Acta, 29, 204-214, 1990), thus providing a further example of the good agreement between experiments and small deformation theory.