Six sewage treatment plants (STP) were investigated over a 12-month period in order to measure the microbiological load of untreated municipal wastewater and to evaluate the removal efficiencies of different treatment systems. The STP investigated can be classified into three categories: bigger plants with tertiary treatment, smaller plants with enhanced secondary treatment, and very small compact facilities. The plants studied had a considerable quantitative impact on the hydrology of the catchment area; consequently, it was anticipated that the microbiological load of the effluent would also be significant. Eighty samples were taken from the influent and effluent of the STP, regardless of weather conditions, and several bacterial and two parasitological parameters were analysed. The average microbiological reduction of each STP was dependent on its capacity and treatment procedures and varied between 1.9 and 3.5log10. Small compact facilities had a significantly lower removal efficiency (2.0+/-1log10) and discharged treated wastewater with a poorer microbiological quality compared to larger plants with tertiary treatment or with enhanced secondary treatment (2.8log10). Final sand filtration and extensive intermediate settling considerably improved the overall microbiological removal efficiency. During the study period, the microbiological water quality of the receiving water course was not significantly impaired by the discharge of any of the investigated plants; however, the compact facilities showed critical treatment deficiencies. In particular, the reduction of Giardia cysts was insufficient (<1.5log10) compared to that of the bigger plants (>3.0log10). In order to quantify the overall impact of microbiological loads on the receiving watercourse in this catchment area, it is also necessary to assess the pollution from combined sewer overflow basins and diffuse pollution. This will be considered in subsequent studies.