Piezoelectric materials exhibit a mechanical response to electrical inputs, as well as an electrical response to mechanical inputs, which makes them useful in sensors and actuators(1). Lead-based piezoelectrics demonstrate a large mechanical response, but they also pose a health risk(2). The ferroelectric BiFeO3 is an attractive alternative because it is lead-free, and because strain can stabilize BiFeO3 phases with a structure that resembles a morphotropic phase boundary(3). Here we report a reversible electric-field-induced strain of over 5% in BiFeO3 films, together with a characterization of the origins of this effect. In situ transmission electron microscopy coupled with nanoscale electrical and mechanical probing shows that large strains result from moving the boundaries between tetragonal- and rhombohedral-like phases, which changes the phase stability of the mixture. These results demonstrate the potential of BiFeO3 as a substitute for lead-based materials in future piezoelectric applications.