We propose that thin films of tetragonal chalcopyrite-structure semiconductor media are promising nonlinearoptical (NLO) materials for the following reasons: Phase matching is possible with them in principle, they can be utilized in a waveguide geometry, and they are compatible with III-V substrates. We investigate the second-order NLO properties of chalcopyrite-structure CuInSe(2) and CuGaSe(2) thin films grown on GaAs(001), using optical second-harmonic (SH) generation at a fundamental wavelength of 790 nm. The SH intensities produced by approximately stoichiometric CuInSe(2) and CuGaSe(2) thin films are similar to that of GaAs. The second-order optical nonlinearities are strongly curtailed in nonstoichiometric thin films.