Aim: This multicenter study was focused on the identification of the microorganisms inhabiting the maxillary sinus and middle nasal meatus in chronic rhinosinusitis. Methodology: 112 middle meatus swabs and 112 maxillary sinus aspirates from 103 patients were available for culture. Results: A total of 244 strains of microorganisms representing more than 50 families were identified in the maxillary sinus and middle nasal meatus (164 and 80, respectively). These included 154 (63.0%) strains of aerobic bacteria from 32 species and 90 (37.0%) strains of anaerobic bacteria from 23 species. Aerobes were more common than anaerobes in both the nasal cavity (78.7% vs. 21.3%) and in the maxillary sinus (55.2% vs. 44.8%). Species of Streptococci (28.8%) and Prevotella (17.8%) were the most common findings in the maxillary sinus aspirates. S. pneumonia, H. influenza, and S. aureus were relatively rare, and found in only 6.7%, 5.4%, and 8.9% of the samples, respectively. Conclusions: The results obtained suggest that common upper airway pathogens do not play a major role in the pathogenesis of chronic rhinosinusitis. The microbiome of inflamed sinonasal mucosa is extremely diverse and involves exotic species of bacteria that, to date, have not been considered as potential inhabitants of the paranasal sinuses.