We report the observation of continuous-wave second-harmonic generation in a modal-phase-matched GaAs/AlGaAs waveguide. The heterostructure has been designed to generate a second-harmonic signal via a type-II process with a fundamental signal around 1.55 mum. Continuous wave conversion enables a quantitative estimation of the conversion efficiency. In our case, eta=30%+/-5% W-1 cm(-2) is found experimentally. (C) 2004 American Institute of Physics.