The formyl radical has been detected via the Ã-X̃ system in a low pressure CH 4/N 2/O 2 flame using cavity ringdown laser absorption spectroscopy. The direct absorption data obtained in these initial studies allows determination of the absolute HCO concentration in the flame of ca. 2.1(±0.9) × 10 13 mol cm −3, which is in excellent agreement with flame chemistry simulations. The extrapolated detection limit at 300 K of ≈ 1.4(±0.6) × 10 11 mol cm −3 is comparable to that of other methods, including intracavity laser absorption spectroscopy.