The object of this paper is to establish an expansion theorem for a regular rightdefinite eigenvalue problem with an eigenvalue parameter )~ which is contained in the Schr6dinger partial differentia}, equation and in a general type of boundary conditions on the boundary of an arbitrary multiply connected bounded domain in R~(n >_ 2). We associate with this problem an essentially self-adjoint operator in a suitably defined Hilbert space and then we develop an associated eigenfunction expansion theorem.