We report the first tunable laser action of a platelet semiconductor in an external cavity. We have achieved cw output powers of up to 9 mW from a CdS platelet optically pumped by an Ar+laser at 476 nm. The energy conversion efficiency is 10% into the TEM00mode. Tunability of the laser output was demonstrated from 495 to 501 nm using an intracavity prism with the crystal kept at a temperature of 95 K. Additional tuning to 504 nm was achieved by varying the temperature to 140 K.