Using aircraft, studies were carried out on the numbers of bacteria and fungi in the air and 10 separate flights were made during 1948 and 1949 covering territory between Goose Bay, Labrador, and Fairbanks, Alaska. Quantitative samples were taken on agar plates, and silicone coated slides in the McGill G.E. Sampler, as well as in glass wool filters and the results correlated with definite air masses. A comparison of methods showed that the McGill G.E. Sampler gives higher and more consistent results for the total viable count than the filters. Silicone coated slides give the total fungus count including nonviable spores and spores that will not grow on the agar plates.From various air masses and during different times of the year, there was considerable variation in the average number of bacteria and fungi found. The highest fungus count was 23.6 per cu. ft. and the lowest 0.01. There was less variation in the number of bacteria, the highest being 5.6 and the lowest 0.2 per cubic foot. The highest fungus counts were found in continental polar air during the summer months while for the bacteria the highest counts were found in Pacific polar air during the spring and fall. The bacteria and fungi isolated were all soil types.