We prove high-frequency modulational instability of small-amplitude Stokes waves in deep water under longitudinal perturbations, providing the first isola of unstable eigenvalues branching off from i 43 . Unlike the finite depth case this is a degenerate problem and the real part of the unstable eigenvalues has a much smaller size than in finite depth. By a symplectic version of Kato theory, we reduce to search the eigenvalues of a 2 × 2 Hamiltonian and reversible matrix which has eigenvalues with nonzero real part if and only if a certain analytic function is not identically zero. In deep water, we prove that the Taylor coefficients up to order three of this function vanish, but not the fourth-order one.
