The extent of reduction in selected microrganisms was tested at a multi-component wastewater treatment plant that treats sewage for a potential re-use in agriculture. The aim of the investigation was to evaluate possible reciprocal correlation among the different microrganisms and to compare the removal of two encysted pathogenic protozoa with that of microbial indicators, Clostridium perfringens spores, enteroviruses and bacteriophages. Samples collected included the raw wastewater, the chlorinated effluent and the effluent after an ultraviolet light treatment. All of the raw sewage samples were positive for Cryptosporidium oocysts and Giardia cysts, as well as for the other microorganisms tested but the bacteriophage B40-8. The data obtained confirm the removal efficiency of the entire process for indicator bacteria but also show the low and variable removal efficiency for the other microbial parameters, such as Giardia and Cryptosporidium, enteroviruses and Clostridium perfringens spores. Reciprocal correlation between Cryptosporidium and Giardia (oo)cysts and the other microbial groups was not demonstrated. The results confirm the resistance of Clostridium perfringens spores, enteroviruses and protozoa to chlorination and demonstrate the relative persistence of these organisms in the effluents even during the ultraviolet light treatment. The yields also emphasise the influence of the analytical method for the determination of protozoan parasites.