Abstract Skeletal and preserved material from 15 piranha fish ( Characidae ) were examined radiographically and the heads of a further three specimens of body length 2.5–4.0 cm examined histologically. Such material revealed a close correlation in the stage of development between upper and lower dentitions on the same side of the jaws but the stage of development of opposite sides of the jaws was out of phase. Histological examination provided evidence that both development and replacement commenced anteriorly, the anterior four teeth in both upper and lower jaws being at an almost identical stage of development. Two explanations were offered to account for the development of this dentition.