An improved condensation nucleus counter (CNC) for use in the stratosphere is described. The University of Minnesota CNC (UMCNC) has a sequential saturator and condenser and uses n-butyl alcohol as the working fluid. The use of a coaxial saturator flow, with aerosol in the center and filtered, alcohol-laden air around it, speeds the response of this instrument and improves its stability as pressure changes. The counting efficiency has been studied as a function of particle size and pressure. The UMCNC provides an accurate measure of submicron aerosol concentration as long as the number distribution is not dominated by sub 0.02 μm diameter aerosol. The response of the UMCNC is compared with that of other stratospherid condensation nucleus counters, and the results of a (near) comparison with a baloon-borne condensation nucleus counter are presented. Ther UMCNC has operated 14 times on a NASA U-2 aircraft at altitudes from 8 to 21.5 km.