The vibrations of a deep slender beam, bent to uniform curvature by invariant moments acting in a vertical plane, which is also the plane of maximum stiffness, have been studied. It is shown that the moments couple up the lateral bending and torsional modes of the beam, those modes being replaced by two independent modes, each involving torsion and flexure. One of these modes is associated with a frequency which decreases with increasing bending moment, the frequency becoming zero when the moment reaches the critical value for lateral instability. The other mode is associated with a frequency which increases with bending moment. Experiments were carried out on an I-section cantilever carrying an end mass. Owing to the varying bending moment, the theoretical analysis of this case is more complicated, and an iterative method, originated by Schwarz (1890), has been employed. Results are in reasonable agreement with experiment.