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An inexpensive GaAlAs single mode semiconductor diode laser operating in the 822 nm wavelength region is frequency stabilised at the line centre of a transition in the (2,1,1) vibration-rotation band of water vapour. A balanced photodetector is used for recording to improve the signal/noise (S/N) ratio. The laser frequency is locked at the line centre by using a simple current-feedback servocircuit. A long-term laser frequency stability of ∼4 MHz is attained, which is more than a hundred-fold improvement over the performance provided by the free running semiconductor diode laser. The corresponding Allan-variance measurement reaches σ(τ)=8.0 × 10-12 for an integration time of 100 s.