Room‐temperature cw operation is reported for an acoustic surface wave amplifier which uses a 25‐&mgr;m‐wide thin film of InSb deposited on LiNbO3. The device also illustrates the use of a thin semiconductor or metal film as a waveguiding structure which exhibits very little dispersion. A cw gain of 23 dB was observed at 340 MHz with a group delay factor (1/&tgr;)∂&tgr;/∂f= −8.3 × 10−7MHz−1.