In order to explore the fundamental nature of hydrofoil flutter, a hydroelastic instability which is a potential cause of structural failure of lifting surfaces operating at high speeds in water, a series of hydrofoil models was tested in a towing basin. Flutter data were obtained from four surface-piercing hydrofoil strut models with various pod and foil configurations. Flutter occurred in two independent hydroelastic modes having predominantly first bending and first torsion mode shapes, respectively. The unstable modes were found to exist simultaneously in at least one strut. Flutter inception speed of the bending flutter mode was affected by strut submergence and the presence or absence of a foil. Flutter inception speed of the torsional flutter mode was dependent on strut cavitation pattern, strut submergence, pod moment of inertia, pod length, and the presence or absence of a foil. (Author).