Large cyclic fluctuations in the densities of laboratory populations of both bacteria and Protozoa were produced by adding sterile nutrient solutions to the cultures at long intervals. The fluctuations were greatly reduced, and they were no longer cyclic when the nutrient solution was added at shorter intervals. Experimentally, the major form of interaction between bacteria and Protozoa was predation of the Protozoa upon the bacteria. Both the population density of the Protozoa and the population density of the bacteria were reduced by decreasing the amount of nutrient solution added to the cultures.