An external cavity diode laser is constructed and shown to have stable, narrow-linewidth operation around 840 nm. Based on this laser and a photoacoustic detector, a gas monitoring system capable of measuring absorption coefficients as low as 10 28 cm 21 is developed. The continuous (mode-hop-free) tuning range of the laser is found to exceed 100 GHz, which makes possible the spectrally resolved detec- tion of water vapor absorption lines. The photoacoustic cell with the ex- ternal cavity diode laser as a light source offers great potential for the development of a portable, automated gas detection system. © 1997 So- ciety of Photo-Optical Instrumentation Engineers.