NCN is detected by laser-induced fluorescence of the A-X transition at 329 nm in low pressure methane-air and methane-nitrous oxide flames. The excitation wavelengths, signal intensities, spatial height profiles above the burner, fluorescence quenching lifetimes, and highly resonant fluorescence wavelength are all consistent with this assignment. The spectral features require the presence of hydrocarbon fuel and nitrogen, and do not gain added intensity under reburn conditions of NO seeding. The resulting arguments and evidence favoring NCN as the initial intermediate in prompt NO formation are discussed. (C) 2002 Elsevier Science B.V. All rights reserved.