The effects of an applied electric field on subband energies and excitonic binding for a graded GaAlAs quantum well are calculated variationally within the effective mass approximation. The very sensitive dependence of subband energies on the applied field is calculated using a model potential profile and exact electron and hole wavefunctions. Our calculations have revealed the dependence of the energy shifts of subbands, and excitonic binding on the field direction in the graded quantum well. This permits control over tunneling which could be desirable for some applications. (C) 1998 Academic Press Limited.