Singlet methylene ((CH2)-C-1) and the formyl radical (HCO) have been studied in a premixed flat flame of CH4 and air by cavity ring-down spectroscopy at I atm. The absorption lines lie in the same spectral region for both species. The (CH2)-C-1 radicals were probed via the (b) over tilde B-1(1) (0,13,0) <--(a) over tilde (1)A(1) (0,0,0) band at 622 nm and the HCO radicals via the (A) over tilde (2)A' (0,9,0) <--(X) over tilde (2)A" (0,0,0) band at 615 nm. Absolute concentrations of (CH2)-C-1 and HCO have been obtained at various heights above the burner and compared to numerical simulations using both the GRI-Mech 2.11 and 3.0 mechanisms, showing relatively good agreement. (C) 2003 The Combustion Institute. All rights reserved.