A B S T R A C TBioaerosols' emission from an activated sludge wastewater treatment plant with surface aerators was surveyed from November 2009 to July 2010. Health effects among plant workers were assessed by checklist and through interviews by an internist. Samples were collected by the Anderson sampler in eight days of the year and analyzed for Standard Plate Count, Total Coliforms, Fecal Coliforms, and Fecal Streptococci. It was found that most sewage treatment plant's staffs were adversely affected as a result of bioaerosols' inhalation. Major observed health effects were fatigue, dizziness, eye irritation, and abdominal pain. The most common size of bacterial-aerosols was higher than 8.2 lm. Maximum and minimum concentrations of bacterial-aerosols were found in the sludge aerobic digester unit (1537 CFU/m 3 ) and the secondary settlers (4 CFU/m 3 ), respectively. Average concentrations of bacterial-aerosols were much lower in summer (742 CFU/m 3 ) as compared to winter (3780 CFU/m 3 ). Given the observed adverse health effects of bioaerosols on employees and their presence in collected air samples, measures to prevent and reduce the spread of bacterial-aerosols in the environment were necessary and the use of personal protection equipments was deemed to be imperative.