In this paper, we deal with single-input single-output systems of the form x(t) = Ax(t) + bu(t) + w y(t) = Cx(t) on a separable Hilbert space H, where the operator A is the generator of an exponentially stable C-0-semigroup on H, b is an element of H, C is a A-admissible linear operator and w is an arbitrary constant disturbance vector in H. We propose a low-gain PI-controller which stabilizes and regulates the system such that, for a given reference constant y(r), y(t) tends to y(r) independently of w as t --> +infinity. 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.