A model was developed that describes the interaction between a pest insect, the house fly,Musca domesticaL., and a wasp,Spalangia endiusWalker, a parasite of house fly pupae. The model is based on life history studies of both insects in the laboratory and in the field. With this model, densities of both insects can be simulated by computer techniques, and the degree of control of house flies that can be obtained by release of the parasite can be studied. The simulation is based principally upon 4 life history parameters: survival of adult females; survival of immature forms; developmental times for immature forms; and fecundity and egg-laying patterns of adult females. Simulations showed that high level control of house flies can be expected by the sustained release of the parasites, a result in agreement with the results of field tests. The simulations were therefore used to study the effects of varying the rate and pattern of parasite releases on house fly density reduction.