Epitaxial PbTiO3 thin films of various thicknesses were prepared by pulsed laser deposition on Pt(001)/MgO(001) substrates. Their ferroelectric domain structures and switching behavior were then investigated mainly using scanning force microscopy as a function of film thickness. Direct evidence of a-domain switching was observed under an external electric field, while its contribution to the total piezoelectric response was found negligible. Thinner epitaxial PbTiO3 films with higher c-domain population and larger tetragonality resulted in enhanced d33 piezoelectric coefficients. The results suggest that the key factor determining the piezoelectric response in epitaxial ferroelectric films is not the a-to-c domain switching but apparently the population of c-domains and their tetragonality. © 2004 American Institute of Physics.