The concentrations and size distributions of airborne microorganisms were determined for outdoor air samples collected with cascade impactors on hazy summer days in Guangzhou. The percentage of airborne bacteria was markedly higher than that of fungi, and the concentrations of bacteria were much higher in a densely populated area (Guangzhou Medical College) than in a clean area (Guangzhou Liuhua Lake Park) or one with heavy traffic (Dongfengxilu Road). Moreover, the concentrations of bacteria at 8:00 and 23:00 were higher than those at 12:00 and 17:00. Sixteen species of bacteria belonging to eight genera were identified, and all were opportunistic pathogens. Of these, 11 species from four genera were Gram-positive, and five species from four genera were Gram-negative. Staphylococcus, Bacillus, Enterobacter, and Serratia were the dominant bacterial genera, and the most abundant species were Staphylococcus hominis (8-58%) and Staphylococcus lugdunensis (14 to 35%). Size distribution studies showed that 36 to 76% of the microorganisms deposited on stages 3 to 6 of the sampler (0.65-4.7 mu m aerodynamic equivalent diameter), and particles of this size could penetrate the lower respiratory tract of humans.