An experiment is performed on a two degree-of-freedom mechanical system having quadratic nonlinearities and linear natural frequencies ω1 and ω2 approximately in the ratio of two-to-one (i.e., ω2 ≈ 2ω1 ). When the lower mode is excited by a harmonic excitation whose frequency Ω is nearly equal to ω1 , amplitude- and phase-modulated responses of the system have been observed for a range of the excitation frequency Ω, in qualitative agreement with the results of a second-order perturbation theory.