none This paper attempts to further analyze and synthesize Professor Utida's classic experiment with the azuki bean weevil, Callosobruchus chinensis , and the braconid wasp, Heterospilus prosopidis (Viereck). First, the density-dependent structure of the host-parasitoid interaction system is examined by the reproduction surfaces of both species. To facilitate the examination, the concept of conditional reproduction curves is introduced. A simple model is then constructed to represent the reproduction surfaces. The fluctuation in the wasp sex ratio is shown to be a major perturbation factor that causes the deviations of data points from the wasp reproduction surface. Finally, a simulation of the system dynamics is attempted, using the estimated density-dependent structure of the system and the hypothesis of perturbation by the variation in the wasp sex ratio. The dynamic pattern thus simulated closely resembles the observed pattern.