This paper characterizes the influence of a wastewater treatment plant with activated sludge aerated by CELPOX devices on the microbiological properties of the air at the plant grounds and in the vicinity. Field studies by the sedimentation method were carried out in seven measurement series in 2001. Sampling posts were each time set with regard to the current meteorological conditions like wind speed and wind direction. Microbiological analyses were comprised of: heterotrophic bacteria at 37şC and 26şC, hemolytic bacteria at 37şC and 10şC, staphylococci mannitol-fermenting and non-fermenting, Escherichia coli, coliforms Pseudomonas, enterococci, actinomycetes, yeasts and moulds. The results indicate that generally - in accordance with the Polish Standards [1,2] the air at the plant grounds and in its vicinity was not polluted. However, few spots (mostly in some distance from the plant) were detected where a group of microorganisms was present in increased numbers which has qualified the air as polluted. Biological aerosols from the activated sludge tanks were not generated in considerable amounts, which was confirmed by the absence or small number of faecal bacteria (coliforms, enterococci), absence of mannitol-fermenting straphylococci and typical water-sewage bacteria of the Pseudomonas fluorescens. The main sources of microbiological pollution were the grit removal chamber and the secondary settlers.