Optical transmission properties are described for two types of waveguides formed in AlxGa1−xAs&sngbnd;GaAs&sngbnd;AlxGa1−xAs epitaxial layers. The first consists of long smooth‐walled mesas formed by a masking and etching procedure and the second are obtained using an additional etching step to selectively etch the GaAs layer. The latter structure is potentially useful in forming active devices such as modulators since this structure is self‐masking for contacting of the top layer by conventional evaporation techniques. The waveguide dimensions are typically 1–30 &mgr;m wide, [sine wave] 1 &mgr;m thick, and several mm in length. The transmission measurements are quite similar for both types of guides with attenuation as low as[inverted lazy s]2 cm−1in wide([inverted lazy s]20 &mgr;m)guides but with losses increasing with decreasing width. The loss appears to arise from imperfections and compositional inhomogeneities in the epitaxial layers.