Femtosecond hyper-Rayleigh scattering (HRS) experiments as a function of amplitude modulation frequency have been performed on tetrathia-[7]-helicene. The apparent first hyperpolarizability beta(app)(omega) is a decreasing function of increasing frequency omega, due to the two-photon fluorescence (2PF) contribution. We observe, however, for the first time, also a frequency-dependent HRS depolarization ratio rho(omega). An accurate value for rho is important, since it offers an indication for the symmetry of the chromophore (5, resp. 1.5, for dipoles, resp. octopoles, but also independent of frequency, since identical for HRS and 2PF). For helicoidal symmetry p(omega) is increasing for higher frequencies, in agreement with theory. (C) 2003 Elsevier Science B.V. All rights reserved.