HCl has been observed in both the troposphere and stratosphere from ground-based and airborne spectroscopic measurements of the 1-0 band at 3 microns wavelength. The results, which are specific to the HCl molecule in the gas phase, show a decreasing mixing ratio with altitude in the troposphere and a rapid increase with altitude in the lower stratosphere. The stratospheric layer, which commences at about 15 km, reaches its maximum concentration at an altitude above 21 km (the limiting height of the observations to date). The local value for the volume mixing ratio at 21 km is 7¿1¿10-10. However, the zenith column abundance observed above 21 km (6.3¿1014 mols⋅cm-2) implies that the mixing ratios at greater altitudes are unlikely to reach values much in excess of the local value at 21 km.