Experiments were conducted to elucidate the effect of host density on sex ratio of parasitoid offspring, using Helicoverpa armigera (Huebner) (Lepidoptera: Noctuidae) and its endoparasitoid Campoletis chlorideae Uchida (Hymenoptera: Ichneumonidae). The sex ratio of Campoletis chlorideae was female-biased showing a linear decrease with increasing parasitoid density at fixed host densities. However, with more hosts available, the number of parasitoids emerging increased, stabilising after a density of 32 hosts per parasitoid. To obtain a female-biased sex ratio, a low density of parasitoids should be released at a recommended site.