The first measurements of ultrafine (3--10 nm diameter) condensation nuclei concentrations and particle size distributions at Storm Peak Laboratory (3210 MSL) are presented. In-cloud average hourly average ultrafine concentrations peaked at 1608 cm-3 in the 1300 hour, MST. Under clear conditions in April and May 2001, average hourly average ultrafine concentrations reached 8947 cm-3 during the 1400 hour, MST. The highest observed 5-minute average ultrafine concentration was 32000 cm-3. A comparison of clear-air and interstitial aerosol size distributions indicates that particles larger than 0.5 ¿m were efficiently removed by snowing, mixed-phase clouds. Particles with diameters between 0.03--0.04 and 0.2 ¿m appeared to serve as cloud condensation nuclei. The observed loss of particles smaller than 16 nm in-cloud may be explained by diffusion to cloud drops.