The aim of this study was to determine the size distribution of bacteria and fungi occurring in the air of human dwellings. The concentration and size distribution of particulate aerosol, Gram-positive mesophilic bacteria, Gram-negative mesophilic bacteria and fungi were examined in 60 flats situated in the Upper Silesia conurbation, southern Poland. The investigated flats comprised three quantitatively equal (20 flats each) groups: flats without additional emission sources of particulate aerosol and microorganisms (Group I), flats with persons who smoke at least one packet of cigarettes per day (Group II), and flats located near steelworks (Group III). The concentrations of four fractions of particulate aerosol were measured by Harvard impactors (PM 2.5 and PM 10) as well as by cyclone HD and 37 mm filter disc holder (PM 5 and TSP). The concentrations of bacteria and fungi were measured by a particle-sizing six-stage Graseby-Andersen impactor. It was found that the concentrations of particulate aerosol in examined flats were below 0.6 mg/m³ and the concentrations of microorganisms were below the level of 10⁴ cfu/m³. The dominant bacteria present in the air of examined dwellings (Micrococcus/Kocuria spp., Staphylococcus spp., Bacillus spp., Pseudomonadaceae, Aeromonas spp., Nocardia spp.) occurred mostly as single particles in the dwellings without additional emission sources, while in the air of dwellings inhabited by tobacco smokers, they often formed aggregates composed of bacterial and dust particles. The fungi dominant in the air of examined dwellings (Penicillium spp., Aspergillus spp., yeasts) occurred mostly as single particles.