The theoretical procedures commonly used by aeroelasticians were applied to predict the flutter speed of a rigid hydrofoil that had two degrees of freedom. The results, compared with corresponding experimental measurements, indicated a discrepancy between theoretical and experimental flutter speeds at low density ratios; the predicted asymptotic behavior of flutter speeds occurred, but at a lower density-ratio. In addition, the accuracy of the circulation terms is more doubtful than that of the added mass and linear terms in the theory.