The interaction of a scalar quantum field with gravity is investigated in the semiclassical context where the space-time is treated classically. It is essentially understood as a self-interaction of the quantum field, mediated by its own states. The relevant states here are not arbitrary but are selected by the principle of equivalence which is incorporated in form of specific nonlinear constraint equations. The quantum field is then subjected to a state dependent (nonlinear) field equation. Concluding, we comment on some problems concerning the consistency of the scheme employed. (orig.).