Microbiological air sampling was performed in a medium-size sewage treatment plant processing municipal wastewater from a city located in eastern Poland. Air samples for determination of the concentrations of viable mesophilic bacteria, Gram-negative bacteria, thermophilic actinomycetes, fungi and endotoxin were collected at 12 sites associated with various phases of sewage treatment process. The concentrations of total mesophilic bacteria (both Gram-positive and Gram-negative) were within arange of 2.4-70.7x10² cfu/m³. Gram-positive coryne form bacteria and cocci were dominant, forming respectively 56.6 % and 24.0 % of the total count. The concentrations of Gram-negative bacteria, thermophilic actinomycetes, and fungi were respectively within ranges of 0.2-5.7x10²cfu/m³, 0-0.5x10²cfu/m³,and 0.24-1.4x10² cfu/m³. Among Gram-negative bacteria, commonly occurred Enterobacter cloacae (17.3% of the total count), followed by Acinetobacter calcoaceticus (16.2%), Pseudomonas spp. (14.0%) and Stenotrophomonas maltophilia (11.1%). Among thermophilic actinomycetes prevailed Thermoactinomyces thalpophilus (47.2%) and Thermoactinomyces vulgaris (22.2%), while among fungi, Geotrichum candidum (32.2%), Penicillium spp. (20%), Cladosporium lignicola (12.2%), and Alternaria alternata (10.4%). Altogether, 20 potentially pathogenic species or genera of bacteria and fungi were identified in the air samples taken in the examined plant. The values of the respirable fraction of airborne microflora varied within a fairly wide range and were between 24.1-100%. The concentrations of airborne endotoxin were in the range of 0.104-5.2ng/m³. In conclusion, the concentrations of microorganisms and endotoxin in the examined municipal sewage treatment plant were low and did not exceed proposed occupational exposure limit values. A moderate risk for the workers may be associated with the presence of potentially pathogenic microbial species having allergenic and/or immunotoxic properties.