We have constructed and tested an optical multiplexer for use with multimode optical fibers. The device utilized a blazed plane reflection grating with a single GRIN‐rod lens, and was made from commercially available components. The wavelength spacing of the two channels was 27 nm, the insertion losses were ⩽2.4 dB, and the crosstalk was <−30 dB. Optimized versions of the device should be capable of providing at least four channels, with still lower insertion losses, in a rugged stable package 1–2 cm long by 2–4 mm in diameter.