The interaction between Asplanchna, predator, and Paramecium, prey, was studied in the laboratory using many experiments of short duration to insure maintenance of a uniform environment over a wide range of population densities. The results of the experiments were used to construct a vector map describing population changes. Regions of uniform vector direction were delineated to produce the isoclines of a graphical model formulated by Rosenzweig and MacArthur (1963). These isoclines were used to predict successfully the instability of this experimental interaction. The effect of time lag in predator response to changed food conditions was incorporated into the model. Time lag increases the instability of this interaction. The relationships between predator and prey and their physiologies determine the shapes of the isoclines. This analysis may be used to help explain other experimental results and naturally occuring situations.