We report the fabrication of a waveguide structure in a near-stoichiometric lithium niobate crystal using 200-MeV argon-ion irradiation at a fluence of 2 × 10(12) ions/cm2. Guided modes were detected in the visible and near-infrared wavelength regions, suggesting that the waveguide can be used at fiber communications wavelengths. The refractive index profiles of the waveguide were reconstructed from the effective index functions. Micro-Raman spectra recorded in the waveguide layer and the substrate showed that the Li/Nb ratio was preserved in the waveguide layer after swift argon-ion irradiation.