Bite performance in lizards influences many aspects of the animal's lifestyle. During feeding, inter- and intrasexual interactions, and defensive behavior, the ability to bite hard might be advantageous. Although biomechanical considerations predict clear relations between head shape and bite performance, this has rarely been tested. Here we investigate the effect of head shape on bite performance in three closely related species of xenosaurid lizards. Our data show that in this family of lizards, bite performance is mainly determined by head height, with high headed animals biting harder than flat headed ones. Species clearly differ in head shape and bite performance and show a marked sexual dimorphism. The dimorphism in head shape also results in an intersexual difference in bite performance. As head height is the major determinant of bite performance in xenosaurid lizards, trade offs between a crevice dwelling life-style and bite performance seem to occur. The evolutionary implications of these results are discussed. J. Exp. Zool. 290:101-107, 2001.