Bacteria and virus-containing aerosols were studied during late summer and fall in a U.S. midwestern suburb before and during the start up and operation of an unenclosed activated sludge wastewater treatment plant. The air in this suburban area contained low-level densities of indicator microorganisms. After the plant began operating, the densities of total aerobic bacteria containing particles, standard plate count bacteria, total coliforms, fecal coliforms, fecal streptococci, and coliphages increased significantly in the air within the perimeter of the plant. Enteric viruses were detected in low densities from the air emissions of this plant. Only standard plate count bacteria remained at significantly higher than baseline densities beyond 250 m downwind from the center of the aeration tanks. Fecal streptococci and coliphages appeared to be more stable in aerosols than the other indicator microorganisms studied. In general, the densities of microorganism containing aerosols were higher at night than during the daytime. The techniques used in this study may be employed to establish microorganism-containing aerosol exposure during epidemiological investigations.