Masses and muscle fibre lengths have been determined for the jaw muscles of Chrysemys (Chelonia). Varanus (Lacertilia) and Caiman (Crocodilia). Hence the force that each muscle can exert has been estimated. The position and direction of the line of action of each force have also been determined. The results are used to calculate the magnitude and direction of the bite forces that each reptile could exert, at particular positions in its mouth, and the magnitude and direction of the reactions at the jaw articulation.