Frequency modulation laser spectroscopy is utilized for high sensitivity detection of trace species aspirated into a flame of the type commonly used for atomic absorption spectroscopy (AAS). The method is applied to the detection of sodium atoms in an air-acetylene flame and data are obtained on the distribution of ground and excited state populations as a function of height above the burner. Absorptions as small as 1.5 × 10-4 may be detected with a signal-to-noise of one. With multi-passing, a detection limit of 3.3 × 106 sodium atoms/cm3 was obtained. © 1983.