1. Previous studies of the Medium Ground Finch, Geospiza fortis, have documented that selection is most severe under drought conditions, which generally favour beaks that are comparatively deep and narrow. Deep beaks are presumed to enhance a bird's ability to crack hard seeds, and narrow beaks have been proposed to enhance a bird's efficiency in manipulating seeds. 2. In the present study, we make the first direct measurements of bite force in Darwin's finches. We used 147 G. fortis from Isla Santa Cruz, Galápagos, to document the influence of beak, head and body dimensions on bite force. 3. Among the various beak dimensions, depth, width and shape were all significant predictors of bite force. Among the various head dimensions, width was the best predictor of bite force. Generally low predictive values of multiple regression models including all morphological variables, as well as positive allometric scaling of bite force on head width, suggest an important additional role for variation in muscle architecture or jaw biomechanics in bite force generation.