In this paper, we deal with single-input single-output systems of the formon a separable Hilbert spaceH, where the operatorAis the generator of an exponentially stable C0-semigroup onH, b ϵ H, C is aA-admissible linear operator andwis an arbitrary constant disturbance vector inH. We propose a low-gain PI-controller which stabilizes and regulates the system such that, for a given reference constantyr, y(t)tends toyrindependently ofwast → + ∞ . Our result generalizes the previous one of Pohjolainen (1982) in that the semigroup is not necessarily holomorphic. A numerical example will be given to illustrate the application of the theory.