The scuticociliate Paranophrys magna Borror,1972 is commonly found in a variety of marine waters , especially in mariculture biotope. The effects of different initial population density, temperature, salinity and pH on the population growth have been studied using experimental methods in previous reports. As the fourth part of this series, the present paper deals with the effects of the competitor (a hypotrichous ciliate) and food on the population growth of Paranophrys magna\%. The experiment was divided ...