Aims: The microbial diversity of bioaerosols released during operational activities at composting plants is poorly understood. Identification of bacteria and fungi present in such aerosols is the prerequisite for the definition of microbial indicators that could be used in dispersal and exposure studies. Methods and Results: A culture-independent analysis of composting bioaerosols collected at five different industrial open sites during the turning of composting piles in fermentation was performed by building 16S rDNA and 18S rDNA libraries. More than 800 sequences were analysed. Although differences in the phylotypes distribution were observed from one composting site to another, similarities in the structure of microbial diversity were remarkable. The same phyla dominated in the five bioaerosols: Ascomycota among fungi, Firmicutes and Actinobacteria among bacteria. For each phylum, some dominant phylotypes were common to at least four bioaerosols. These common phylotypes belonged to Thermomyces, Aspergillus, Penicillium, Geobacillus, Planifilum, Thermoactinomyces, Saccharopolyspora, Thermobifida and Saccharomonospora. Conclusions: The microbial signature of aerosols produced during the thermophilic phase of composting was determined. The similarities observed may be explained by the selection of thermophilic and sporulating species. Significance and Impact of the Study: Several bacteria and fungi identified in this study may represent potential indicators of composting bioaerosols in air.