Small aquatic Peritrichida are the typical protozoa found frequently in the activated sludge and bio-film from wastewater treatment system. Effect of environmental factors on growth characteristics of aquatic Peritrichida, Vorticella microstoma and Vorticella convallaria were investigated. Using a sterilized activated sludge as the protozoan food, the effects of temperature, pH, phosphate buffer solution concentration, shake stress and food concentration on the monoxenic growth rate of small Peritrichida were measured. Optimum temperature of V. microstoma and V. convallaria were 25°C. Phosphate buffer concentrations which could grow normally were 1/75M for each Peritrichida. The optimum pH values ranged from 5.7 to 7.7 for V. microstoma and ranged from 4.7 to 8.7 for V. convallaria. The endurance against shake stress was lower than those of Rotatoria and Oligochaeta.