Long-path measurements of carbon monoxide in the atmosphere are described. The technique of resonance absorption was used in which the wavelength of radiation from a PbS(0.82)MbSe(0.18) semiconductor diode laser was tuned into coincidence with an absorption line of CO in its fundamental band around 4.7 microm. By employing rapid frequency modulation of the laser emission to overcome atmospheric turbulence effects, it was possible to achieve a minimum detectable concentration of 5 parts per billion over a 0.61-km path. Continuous around-the-clock monitoring was also performed and permitted increases in the ambient CO level due to commuter traffic to be observed.