abstract: We consider the fundamental invariant of a real hypersurface in CN-its holomorphic symmetry group- and analyze its structure at a point of degenerate Levi form. Generalizing the Chern-Moser operator to hypersurfaces of finite multitype, we compute the Lie algebra of infinitesimal symmetries of the model and provide explicit description for each graded component. Compared with a hyperquadric, it may contain additional components consisting of nonlinear vector fields defined in terms of complex tangential variables. As a consequence, we obtain exact results on jet determination for hypersurfaces with such models. The results generalize directly the fundamental result of Chern and Moser from quadratic models to polynomials of higher degree.