Abstract The outdoor aeromycota was studied in Waterloo, Ontario, Canada throughout 1992. The dominant outdoor airborne fungal genera and groups recorded on 58 sampling dates were Cladosporium (41.0%), unidentified basidiospores (12.1%), unidentified spores (8.5%), Ganoderma (7.2%), unidentified ascospores (5.3%), Leptosphaeria (5.3%), Coprinaceae (5.0%), hyphal fragments (4.9%), Aspergillus/Penicillium (3.8%), Alternaria (1.8%), and Epicoccum (1.3%). Most common outdoor genera showed distinct diumal periodicities from May to October, with the exception of Aspergillus/Penicillium. Most hyphomycetes displayed daytime peaks, except for Aspergillus/Penicillium. Ascospores of Leptosphaeria had morning peaks, and basidiospores of Ganoderma and Coprinaceae had early morning patterns. Most common outdoor genera, except for Aspergillus/Penicillium, also displayed well defined seasonal patterns, with peak periods between May and October.