Nonlinear energy transfers with sea and swell (frequencies 0.05--0.40 Hz) were responsible for much of the generation and loss of infragravity wave energy (frequencies 0.005--0.050 Hz) observed under moderate- and low-energy conditions on a natural beach. Cases with energetic shear waves were excluded, and mean currents, a likely shear wave energy source, were neglected. Within 150 m of the shore, estimated nonlinear energy transfers to (or from) the infragravity band roughly balanced the divergence (or convergence) of the infragravity energy flux, consistent with a conservative energy equation. Addition of significant dissipation (requiring a bottom drag coefficient exceeding about 10-2) degraded the energy balance.