Air samples collected at several heights above the Arctic Ocean contained at least two types of bacteria and five different fungal spores which could elevate the freezing temperature of water droplets. It was observed that the freezing temperture increased logarithmically with the number of cells per drop up to a maximum temperature dependent upon species. Bacteria were more effective than fungal spores but, unlike the spores, they lost their activity under laboratory growth conditions. Maximum concentration of nearly 10 microbial cells per liter occurs in clouds and 1 per liter in air outside clouds. However, numbers and varieties of microbial cells in excess of those found near sea ice surface level were observed up to 7 km.