The flexural modes of a circular steel disk (diameter ≈25.7 cm, thickness ≈0.061 cm) were excited by an electromagnetic force to large amplitudes of motion, and the nonlinear tuning curves of the (p,q) modes, wherep= nodal diameters andq= nodal circles, were determined. The results disagree with those of S. A. Tobias [Engineering51(11 July 1958) and Proc. Inst. Mech. Eng. London171, 691 (1957)]. It is found that, as the drive level is increased, the peak frequencies of the tuning curves can shift to lower frequencies, such as (0,1), (1,1), (0,2) modes, or higher frequencies, such as (10,0), (3,1), (4,0) modes; the direction of shifting is a property of the particular modes being driven. [Work supported by ONR and DOE.]