We present the results of investigations of microorganisms present in aerosol emitted by selected devices of two mechanical-biological wastewater treatment plants (“Debogorze” and “Gdańsk-Wschód”). In 2002, 5 series of measurements were conducted at each of the plants. The samples of air were collected by means of sedimentation method (PN-89/Z-04008/08) and by means of filtration method, using an air sampler produced by Merck. Apart from measurements of microbiological analyses, consisting of determinations of the total number of psychrophillic and mezophillic bacteria, manitol-positive and manitol-negative staphylococcus, hemolysing staphylococcus, Pseudomonas fluorescens, coliform bacteria and the number of spores of phycomycetes, weather conditions were also monitored. The most numerous microorganisms in the monitored air were psychrophillic bacteria and spores of phycomycetes. The number of psychrophillic bacteria varied in the wide range from 14 to 5255 CFU/m³, the number of mezophillic bacteria changed from 1 to 1324 CFU/m³, the number of staphylococcus – from 1 to 150 CFU/m³, the numbers of Pseudomonas fluorescens and the coliforms varied from a few to 50 CFU/m3 and the spores of phycomycetes varied from a few to 5,250 CFU/m³ of air (collected by means of Merck air sampler). It was found out that in both plants the main sources of microorganisms were sludge recirculation chambers and sand traps. The air around aeration chambers of activated sludge contained similar numbers of microorganisms as the background. It was proved that the sampling method is of critical effect on measurement results. The number of microorganisms detected in the samples collected by means of sedimentation method was by one order of magnitude higher than in the samples collected by means of filtration method (using air sampler), allowing for strict control of the filtered air volume.