After growing mass and monoxenic cultures of three species of small metazoa, Nais sp., Aeolosoma hemprichi and Philodina sp. using sterilized activated sludge as the metazoan food, the effect of six kinds of metals on the specific growth rate of each metazoa was examined. EC₅₀ (effective concentration) was used to define the metal concentration required to reduce the specific growth rate of metazoa to half of that of a control. The EC₅₀ for A. hemprichi, Nais sp., and Philodina sp. were: 0.9, 1.3 and 5.4 mg · l^-1, respectively for copper, 0.04, 0.03 and 1.4 mg · l^-1 for hexavalent chromium, 3.4, 4.3 and 9.2 mg · l^-1 for zinc, 14.0, 7.6 and 33.0 mg · l^-1 for manganese and more than 102.4 mg · l^-1 under pH adjusted (A. hemprichi and Nais sp.) for alminium and iron. It was noted that concentrations of these metals from the influent of domestic sewage treatment plants were apparently less than values of EC₅₀ mentioned above.