The ultrasensitivity of the cavity ring down spectroscopy technique has been used to detect the minor CN species in a premixed rich (φ=1.2) methane/air flame stabilized at 53 hPa. The CN radical has been probed around 387 nm on the B–X (0–0) band. A peak concentration of 4.7·109 cm−3 has been measured which corresponds to a mole fraction of 0.022 ppm. A detection limit of a few ppb has been reached despite the presence of an important off-resonance background due to the flame.