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This study investigated the effects of host density and distribution on the patch-leaving behavior of Diadegma semiclausum (Hymenoptera: Ichneumonidae), a solitary endoparasitoid of larval Plutella xylostella (Lepidoptera: Plutellidae). Individual female wasps were released onto an experimental plant infested with host larvae at different densities and distributions, and were allowed to freely leave for an alternative host plant placed upwind of the experimental plant in a wind tunnel. The influence of host density and distribution, as well as within-patch foraging experience, on the parasitoid's patch-leaving tendency was analyzed by means of the proportional hazards model. This study aimed to test the predictions of a number of patch-leaving models, including the Marginal Value Theorem, “rules of thumb,” and incremental or countdown mechanisms. The parasitoid's patch-leaving tendency decreased with increased host density, more clustered host distribution, and unsuccessful host encounter as a result of host defense, but increased with successful oviposition. None of the simple rules of thumb such as fixed time, fixed number of hosts parasitized, or fixed giving-up time was employed by this parasitoid. The results agreed with the general predictions of the Marginal Value Theorem that patch residence time and numbers of ovipositions by the parasitoid increased with increasing host density. The decreasing influence of oviposition on the parasitoid's patch-leaving tendency, regardless of host density or distribution, was consistent with the prediction of a countdown mechanism.