SUMMARYAirborne fungus spores were studied for numbers and viability by hour-long samples from 5 AM to 9 PM in 2 series; the first on 20 days from June to September, 1959, and from June to mid-August, 1960, the second on 17 consecutive days from August 27 to September 12, and October 9. Four spore types were studied:Alternaria, Cercospora, Cladosporium, and fusiform ascospores. Spores were obtained on silicone-coated slides exposed in a slit sampler. Viability was tested by incubating spores 48 hr in a moist chamber.Diurnal periodicity was observed in spore numbers of all 4 fungi:Alternariahad peaks 10–40/ft3, occasionally higher, at 11 AM and 6 PM;Cercosporahad similar peaks but numbers fewer than 10/ft3;Cladosporium, usually over 100/ft3, was variable, but twin peaks at 11 AM and 6 PM predominated; fusiform ascospores were less than 10/ft3with a nighttime peak.Spore germination inAlternariaaveraged 80% and was constant throughout the day. InCercosporagermination averaged 58% but was low at morning and night, increasing to a peak of 70–90% at 11 AM then declined.Cladosporiumand fusiform ascospores averaged 42 and 55%, respectively, with viability decreasing in the morning to a midday low and increasing in the afternoon and evening.