The concentration of telluric carbon monoxide has been determined in the upper stratosphere from infrared solar spectra recorded by balloon on October 1978 and September 1979, over Texas, USA. The average mixing ratios were found to be (2.1∓.3)¿10-8 ppv and (3.7∓.4)¿10-8 ppv above the float altitudes of 30.6 and 36.8 km, respectively. The concentration profile deduced from all our measurements indicates that the CO mixing ratio increases in the upper stratosphere, from (∓.3)¿10-8 ppv at 30.6 km, to about 4¿10-8 ppv above 41 km.