Photocurrent and photoluminescence excitation spectra of GaAs/AlxGa1-xAs quantum wells in an electric field exhibit two exciton peaks in the energy range where only a single exciton is expected to occur. Uniaxial stress and polarization-dependent measurements indicate that the two peaks have strongly mixed heavy- and light-hole characters. The observation of an additional peak is attributed to mixing between the n=2 heavy-hole and n=1 light-hole valence subbands. Our measurements provide direct evidence for the large effect that valence-band mixing has on excitons in quantum wells. © 1987 The American Physical Society.