none Compsilura concinnata; Epicampocera succincta; LEPIDOPTERAN-HOSTS; Pieris melete & P. rapae; Inter & intraspecific competition within host; COMPETITION-; Larval interactions within lepidopteran host Pieris melete; DIPTERAN-PARASITES; Compsilura concinnata & Epicampocera succincta; Inter & intraspecific competition within host Pieris rapae; DIPTERAN-PARASITES; Compsilura concinnata & Epicampocera succincta; HYMENOPTERAN-PARASITES; Cotesia glomerata; Inter & intraspecific competition within host Cotesia glomerata; LEPIDOPTERAN-HOSTS; Pieris rapae; Inter & intraspecific competition within host; COMPETITION-; Larval interactions within lepidopteran host Intra- and interspecific larval interactions that take place in a host body were investigated for two tachinid flies Epicampocera succincta and Compsilura concinnata (Diptera: Tachinidae) parasitizing Pieris butterfly larvae. E. succincta, a specialist on Pieris butterflies, showed contest-type intraspecific competition, eliminating all the other conspecific larvae. On the other hand, an extreme generalist parasitoid C. concinnata exhibited scramble-type competition, sharing the host with other conspecifics and suffering reduced body size as a result. However, when these two species occurred together in a single host, C. concinnata had a much higher chance of survival. Moreover, C. concinnata could often survive in the presence of a parasitoid wasp Cotesia glomerata (Hymenoptera: Braconidae) while E. succincta could not. The high tolerance of C. concinnata could be attributable to its being an extreme generalist: To attack and survive on many different hosts, one has to be able to deal with various competitors. The competitive inferiority of the specialist E. succincta, on the other hand, may be a result of relatively recent encounter with those competitors.