Abstract: To examine the effectiveness of the multiple-species microbial community test for monitoring toxic stress, the artificial substrate, polyurethane foam unit (PPU), method developed by Dr. Cairns was taken for colonizing mierobial community. Field and laboratory experiments of toxic effects on the community were conducted in a series of five oxidation ponds (in Ya-Er Lake.) built for treating wastewater from a pesticide factory in Sep.-Dec. 1982. The microbial community structural parameters (protozoan species composition, chlorophyll component and bacterial abundance) and the functional parameters (protozoan colonization rate and community respiration rate) were estimated. Concentrations of organic phosphorus, p-nitrophenol, HCH, COD in the ponds were analysed as well. Both structural (and functional parameters appeared to be good indicators of stress. The higher concentrations of the toxicants, the lower was the colonization rate of the protozoan community. The results from the field were found to be coincident with that from the laboratory. Therefore, it is possible to predict the effects of various chemicals on the ecosystems with the PFU protozoan community toxicity tests. The number of protozoan species colonizing PFU after the exposure of 1 to 3 days is good enough for a routine biological monitoring. All the experiments were repeated successfully by students in the PFU training course in 1983 and 1984. It is justified that the PFU method is a rapid, accurate and economic way in biological monitoring.