Abstract: To investigate the influence of rearing density of host larvae of Loxostege sticticalis on parasitic efficiency of tachinid parasitoids, Exorista civilis Rondani, parasitism rate and host tolerance were studied at three rearing densities (1, 10 and 30 larvae per jar) in the laboratory (22℃,RH 70%,photoperiod 16L:8D). The results showed that parasitism rate and larval tolerance were significantly affected by larval rearing densities. Parasitism rate was the highest at the rearing density of 30 host larvae per jar and was significantly different between the three treatments. Superparasitism rate increased with larval rearing density, being significantly higher at 30 larvae per jar than at 1 larva per jar. Larval tolerance to parasitism was to the contrary of parasitism rate. Mortality of parasitized larvae was the lowest at 10 larvae per jar, which was significantly lower than that at 30 larvae per jar. Survival rate of tachinid fly at 10 larvae per jar was significantly lower than those at 1 and 30 larvae per jar. The results indicate that host larval rearing densities is one of the major factors influencing parasitic efficiency of E. civilis and clarify the relationship between fly parasitism rate and host rearing density.