The development of a condensation nucleus counter (CNC) for detecting ultrafine aerosol (UFA) particles (particle diameter ≤20 nm) is described here. This instrument is designed specifically for high-efficiency detection and counting of ultrafine particles. It is a modified version of the TSI model 3020 continuous flow, single-particle-counting condensation nucleus counter with sequential saturator and condenser. Design modifications incorporating aerosol sheathing techniques have minimized inlet sampling losses and optimized nucleus detection efficiency. Overall counting efficiency for the UFACNC is theoretically predicted including inlet sampling, nucleus activation, and droplet detection efficiencies. The theory was experimentally verified for low concentration, mono-disperse NaCl aerosols in the 2 to 15 nm particle diameter range. The UFACNC has a counting efficiency of > 70% for particles down to 3.5 nm in diameter. Below this size counting efficiency drops sharply to zero at about 2.6 nm with 50% efficiency at about 3 nm.