We investigate the second order optical susceptibility &khgr;(2)of symmetric GaAs/AlGaAs quantum wells under applied bias, in the frequency region near half the band gap. The second harmonic is generated in a transmission geometry, and is separated from bulk contribution by a lock‐in detection technique. As opposed to previous reports, we find that &khgr;xzx(2)is much higher than &khgr;zxx(2). For 50 A˚ quantum wells, &khgr;xzx(2)≊25 pm/V and &khgr;zxx(2)≊0 at &lgr;=1.66 &mgr;m under an applied electric fieldE=50 kV/cm. ©1995 American Institute of Physics.