Two constructed wetlands were evaluated regarding their performance in treating different wastewaters under the tropical conditions of Thailand. The first one was for treating the domestic sewage from a governmental institute, whereas the latter was used to improve the quality of the effluent from a central aerated lagoon, for which the influent had been already pretreated by about 100 factories in an industrial estate. The hydraulic loading rate and the hydraulic retention time were 0.015 m3/m2.d and 28 days and 0.3 m3/m2.d and 3 days for the two wetland systems, respectively. The physical and chemical water quality and the occurrences of protozoa and metazoa were monitored for three seasons, each of which had samples taken twice a week for two months. The temperature was quite constant throughout the whole year, ie., 26 to 31°C and 25 to 34°C for the two wetlands, respectively. The water quality of the wetland effluents was rather good; the BOD and TKN were only 4.5 and 3.3 mg/l and 2.7 and 6.8 mg/l for the two cases. A relationship between the filtered BOD and the zoological characteristics was established. As a result, a simple glance at the occurrence of certain metazoa and protozoa can rapidly but approximately predict the water quality of the system.