Continuous-wave 640 nm operation at room temperature (20 degree C) has been achieved by a transverse-mode stabilized AlGaInP laser diode with an (Al//0//. //1//5Ga//0//. //8//5)//0//. //5In//0//. //5P quaternary active layer. The lasing wavelength is claimed to be the shortest ever reported for room-temperature cw operation of semiconductor lasers. The laser structure was grown by metalorganic vapor-phase epitaxy. The threshold current (density) was 63 ma (4. 2 ka/cm**2).