Continuous wave mid-infrared radiation at 3.36 mum (power approximate to 18 muW) has been produced by difference frequency generation of the fundamental of a YAG laser at 1064 nm (220 mW) with a fibre pigtailed DFB diode laser operating at 1560 nm (50 mW) in periodically poled lithium niobate. The idler radiation has been used to make real-time measurements of ambient methane present in our laboratory using wavelength modulation spectroscopy via the P(4) multiplet transitions within the v(3) band. We demonstrate a minimum detectable concentration of 47 ppb over a path length of 0.71 m in a 1 s observation time. Measurements of the pressure broadening and shift parameters for these lines are presented for methane in the presence of air, nitrogen and argon. (C) 2004 Elsevier B.V. All rights reserved.