Microbiological air sampling was performed in a medium-size sewage treatmentplant processing municipal wastewater from a city located in eastern Poland. Air samples for determinationof 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 2.4-70.7 x 10(2) cfu/m(3). Gram-positive coryneform bacteria and cocci were dominant, formingrespectively 56.6 % and 24.0 % of the total count. The concentrations of Gram-negative bacteria, thermophilicactinomycetes, and fungi were respectively within ranges of 0.2-5.7 x 10(2) cfu/m(3), 0-0.5 x 10(2) cfu/m(3),and 0.24-1.4 x 10(2) cfu/m(3). Among Gram-negative bacteria, commonly occurred Enterobacter cloacae (17.3% of the total count), followed by Acinetobacter calcoaceticus (16.2 %), Pseudomonas spp. (14.0 %) andStenotrophomonas maltophilia (11.1 %). Among thermophilic actinomycetes prevailed Thermoactinomyces thalpophilus(47.2 %) and Thermoactinomyces vulgaris (22.2 %), while among fungi, Geotrichum candidum (32.2 %), Penicilliumspp. (20 %), Cladosporium lignicola (12.2 %), and Alternaria alternata (10.4 %). Altogether, 20 potentiallypathogenic species or genera of bacteria and fungi were identified in the air samples taken in the examinedplant. The values of the respirable fraction of airborne microflora varied within a fairly wide rangeand were between 24.1-100 %. The concentrations of airborne endotoxin were in the range of 0.104-5.2ng/m(3). In conclusion, the concentrations of microorganisms and endotoxin in the examined municipalsewage treatment plant were low and did not exceed proposed occupational exposure limit values. A moderaterisk for the workers may be associated with the presence of potentially pathogenic microbial specieshaving allergenic and/or immunotoxic properties.